Display device

ABSTRACT

The present invention is intended to suppress power consumption of an EL display. In accordance with the brightness of an image to be displayed in a pixel portion, the contrast of the image is determined whether to be inverted or not, and the number of bits of the digital video signal to be input into the pixel portion is reduced, and the magnitude of a current to flow through the EL element is allowed to be maintained at a constant level even when a temperature of an EL layer changes by providing the EL display with another EL element to be used for monitoring a temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 09/833,673, filed Apr. 13, 2001 now U.S. Pat. No. 7,221,338, nowpublished as US-2001-0033252-A1, which claims the benefit of foreignpriority applications filed in Japan on Apr. 18, 2000, as Serial Number2000-117045, and on Mar. 5, 2001, as Serial Number 2001-059511. Thisapplication claims priority to each of these prior applications, and thedisclosures of the prior applications are considered part of (and areincorporated by reference in) the disclosure of this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display panel in which a lightemitting element formed on a substrate is sealed between the substrateand a cover member. In addition, the present invention also relates to adisplay module in which an IC chip is mounted on the above-mentioneddisplay panel. It should be noted that in the present specification, thedisplay panel and the display module are collectively referred to as thelight emitting device. The present invention further relates to anelectronic apparatus that employs the above-mentioned light emittingdevice.

2. Description of the Related Art

Recently, techniques for forming a TFT on a substrate has been greatlyadvanced, and much developments have been made to apply those techniquesto an active-matrix type display device. In particular, a TFT employinga poly-crystalline silicon film can operate at a higher speed since afield effect mobility (simply referred to as the mobility) thereof islarger than that of a TFT employing the conventional amorphous siliconfilm. Thus, it becomes possible to control pixels by means of a drivercircuit formed on the same substrate as the pixels. Such the pixels wasconventionally controlled by means of a driver circuit provided at theoutside of the substrate.

The active-matrix type display device as mentioned above can exhibitvarious advantages such as a reduced fabricating cost, miniaturizationof the display device, an increased fabricating yield, a reducedthroughput or the like, by providing various circuits and elements onthe identical substrate.

Furthermore, developments of an active-matrix type light emitting devicehaving light emitting elements as a self-emission type element have beenactively conducted. Such a light emitting device is also referred to asan Organic EL Display (OELD) or an Organic Light Emitting Diode (OLED).

Unlike a liquid crystal display, the light emitting device is of theself-emission type. The light emitting element has a structure in whicha layer containing an organic compound (hereinafter referred to as theorganic compound layer) that allows luminescence to be generated byapplying an electrical field thereto is interposed between a pair ofelectrodes (an anode and a cathode). The organic compound layer usuallyhas a layered structure. Typical examples therefor include a layeredstructure of “a hole transportation layer/a light emitting layer/anelectron transportation layer” proposed by Tang et al. of Eastman KodakCo. This structure has a high luminous efficiency, and most of lightemitting devices about which research and development activities arecurrently being progressed employ this structure.

Although a luminescence in an organic compound includes a singletexcitation (fluorescence) and a triplet excitation (phosphorescence),the light emitting device of the present invention can use one or bothof the above luminescence.

Alternatively, a layered structure in which a hole injection layer/ahole transportation layer/a light emitting layer/an electrontransportation layer, or a hole injection layer/a hole transportationlayer/a light emitting layer/an electron transportation layer/anelectron injection layer are formed on an anode in these orders may beused. Furthermore, fluorescent dyes or the like may be doped into thelight emitting layer.

In the present specification, all of the layers to be disposed betweenthe cathode and the anode are collectively referred to as the organiccompound layer. Accordingly, all of the above-mentioned layers such asthe hole injection layer, the hole transportation layer, the lightemitting layer, the electron transportation layer, the electroninjection layer or the like are included in the organic compound layer.

A predetermined voltage is applied to the organic compound layer made ofthe above-mentioned structure via the pair of electrodes, and thusrecombination of carriers occurs in the light emitting layer, therebyresulting in light emission. In the present specification, when thelight emitting element emits light, the light emitting element isexpressed as being driven. In addition, in the present specification,the light emitting element composed of an anode, an organic compoundlayer, and a cathode is referred to as the light emitting element.

Since the light emitting device is not required to employ a back light,a thickness and a weight of the display itself can be suppressed, ascompared to a liquid crystal display. For that reason, the lightemitting device has come to be used as a display section of a portableinformation terminal (a mobile computer, a portable telephone, aportable game apparatus, an electronic book or the like), instead of theliquid crystal display.

In order to suppress power consumption of the portable informationterminal, it has been desired to suppress power consumption of the lightemitting device to be employed as the display section.

Moreover, digitalization of a receiver or a VTR to be used in home aswell as various equipment to be used in a TV or radio broadcast stationhas now proceeded. As the next step following digitalization of abroadcast system is digitalization of broadcast electric waves, i.e.,realization of digital broadcasting. Vigorous research and developmentactivities have been performed toward such a goal.

One scheme to be used for realizing a digital driving of the lightemitting device is a temporal gradation display. More specifically, thetemporal gradation display refers to a driving method in which thegradation display is performed by controlling a time period during whichthe light emitting element is to emit light in one frame period.

In the case where the light emitting device is digitally driven toperform the temporal gradation display, the required number of rewritinga digital video signal containing image information to be input topixels is increased, as the number of gradation of an image to bedisplayed is increased. Consequently, power consumption of a group ofdriver circuits to be used for inputting the digital video signal intothe pixels is increased, thereby resulting in increased powerconsumption of the light emitting device.

In addition, since the light emitting element is of the self-emissiontype, a time period during which the light emitting element is to emitlight in one frame period varies depending on an image to be displayed.Accordingly, power consumption of the light emitting device in turnvaries depending on an image to be displayed.

Furthermore, the magnitude of a current to flow through the lightemitting element varies also depending on temperature. Even with avoltage of the same magnitude being applied across the electrodes of thelight emitting element, the magnitude of a current to flow through thelight emitting element varies due to temperature characteristics of thelight emitting element. For example, as the temperature of the organiccompound layer increases, a larger current tends to flow. Accordingly,as an environmental temperature under which the light emitting elementis to be used increases, power consumption of the light emitting deviceincreases, which in turn increases the brightness of the light emittingelement.

In view of the above-described situations, the present invention isintended to suppress power consumption of a light emitting device and anelectronic apparatus employing the light emitting device as its displaysection.

SUMMARY OF THE INVENTION

A first structure according to the present invention is characterized inwhich in the case where a monochrome display is performed in a lightemitting device, the contrast of an image is determined whether to beinverted or not in accordance with what kind of an image to be displayedby pixels in a pixel portion.

In accordance with the above-mentioned structure, the magnitude of acurrent to flow through the light emitting element can be suppressed tosome degree, thereby resulting in a reduced power consumption of thelight emitting device.

Furthermore, a second structure in accordance with the present inventionis characterized in that in a light emitting device that performs adigitally-driven time-divisional gradation display, a digital videosignal input to a source signal line driver circuit included in thelight emitting device is input into a pixel portion after its number ofbits is reduced. More specifically, a bit of the digital video signal issequentially cut off from the least significant bit to reduce the numberof bits of the digital video signal to be input into the pixel portion.

In accordance with the above-mentioned structure, since the number ofbits of the digital video signal to be input into the pixels is reduced,the required number of writing the digital video signal by the sourcesignal line driver circuit and a gate signal line driver circuit can bereduced. Thus, power consumption of the source signal line drivercircuit and the gate signal line driver circuit can be reduced, therebyresulting in a reduced power consumption of the light emitting device.

Moreover, in accordance with a third structure of the present invention,a light emitting device is provided with a light emitting element formonitoring a temperature. One of electrodes of thetemperature-monitoring light emitting element is connected to a constantcurrent source. Thus, by means of temperature characteristics of thetemperature-monitoring light emitting element, the magnitude of acurrent to flow through a light emitting element of a pixel ismaintained at a constant level.

In accordance with the above-described structure, the magnitude of acurrent to flow through the light emitting element of a pixel ismaintained at a constant level, even when a temperature of the organiccompound layer changes. Thus, power consumption of the light emittingdevice can be prevented from increasing even when an environmentaltemperature of the light emitting device increases, which in turn canmaintain the brightness at a constant level.

In accordance with the first, second, and third structures of thepresent invention as described in the above, it is possible to suppressthe power consumption of a light emitting device and an electronicapparatus which employs the light emitting device. It should be notedthat only either one of the first through third structures is requiredto be included in the present invention, although two or all of thefirst through third structures may be included.

Structures in accordance with the present invention will be furtherdescribed below.

The present invention provides a display device having a plurality ofpixels, wherein a polarity of a digital video signal to be input intothe plurality of pixels is inverted, thereby resulting in brightness ofthe plurality of pixels being changed.

The present invention provides a display device having a pixel portionincluding a plurality of pixels, and a source signal line drivercircuit, wherein the source signal line driver circuit includes aswitching circuit for switching a polarity of an output, and a polarityof a digital video signal input to the switching circuit is inverted bymeans of a shift signal to be input into the switching circuit and theresultant signal is then input into the plurality of pixels.

The present invention provides a display device having a pixel portionincluding a plurality of pixels, and a source signal line drivercircuit, wherein each of the plurality of pixels has a light emittingelement, and the source signal line driver circuit includes a shiftregister, one or more latches and a switching circuit, and a polarity ofa digital video signal input from the one or more latches into theswitching circuit is inverted by means of a shift signal to be inputinto the switching circuit and the resultant signal is then input intothe plurality of pixels.

The present invention provides a display device having a pixel portionincluding a plurality of pixels, and a source signal line drivercircuit, wherein each of the plurality of pixels has a light emittingelement, and the source signal line driver circuit includes a shiftregister, one or more latches and a switching circuit, and a polarity ofa digital video signal input from the one or more latches into theswitching circuit is inverted by means of a shift signal to be inputinto the switching circuit and the resultant signal is then input intothe plurality of pixels, and an average of a time period during whichall of the light emitting elements emit light in one frame period isequal to or less than a half of the maximum value of the time periodduring which all of the light emitting elements emit light in one frameperiod.

The above structures may be characterized in which the switching circuitincludes an inverter, a first analog switch, and a second analog switch,wherein the digital video signal input into the switching circuit isinput into an input terminal of the first analog switch via theinverter, the digital video signal output from the one or more latchesis input into an input terminal of the second analog switch, the shiftsignal is input from a first control input terminal of the first analogswitch and a second control input terminal of the second analog switch,a signal obtained by inverting the polarity of the shift signal is inputfrom a second control input terminal of the first analog switch and asecond control input terminal of the first analog switch, and signalsoutput from output terminals of the first analog switch and the secondanalog switch are output from the switching circuit.

The above structures may be characterized in which the switching circuitincludes an inverter, a first NAND, a second NAND, and a NOR, whereinthe first NAND is supplied with the digital video signal via theinverter and the shift signal, the second NAND is supplied with thedigital video signal and a signal obtained by inverting a polarity ofthe shift signal, a signal output from the first NAND and a signaloutput from the second NAND are input into the NOR, and a signal outputfrom the NOR is output from the switching circuit.

The present invention provides a display device having a plurality ofpixels and a source signal line driver circuit, wherein among a digitalvideo signal to be input into the source signal line driver circuit,only more significant bits are input into the plurality of pixels.

The present invention provides a display device having a pixel portionincluding a plurality of pixels, and a source signal-line drivercircuit, wherein the source signal line driver circuit includes a shiftregister, a first latch, a second latch, and a clock signal controlcircuit, a clock signal is input into the shift register via the clocksignal control circuit to thereby output a timing signal from the shiftregister, a digital video signal is input into and held at the firstlatch by the timing signal, the digital video signal held at the firstlatch is input into and held at the second latch by a latch signal, thedigital video signal input into and held at the second latch is inputinto the plurality of pixels, and the clock signal control circuitreduces the number of bits of the digital video signal to be input intoand held at the first latch by supplying a constant fixed electricalpotential instead of the clock signal to the shift register a constantperiod of time.

The above structure may be characterized in which the clock signalcontrol circuit includes a NAND and an inverter, wherein a clock signaland a selection signal are input into the NAND, and a signal output fromthe NAND is output from the clock signal control circuit via theinverter.

The above structure may be characterized in which the clock signalcontrol circuit includes a first analog switch, a second analog switch,and an inverter, wherein a selection signal is input via the inverterinto a second control input terminal of the first analog switch and afirst control input terminal of the second analog switch, the selectionsignal is input into a first control input terminal of the first analogswitch and a second control input terminal of the second analog switch,a clock signal is input into an input terminal of the first analogswitch, a fixed electrical potential is supplied to an input terminal ofthe second analog switch, and signals output from output terminals ofthe first analog switch and the second analog switch are output from theclock signal control circuit.

The present invention provides a display device having a pixel portionincluding a plurality of pixels, and a source signal line drivercircuit, wherein the source signal line driver circuit includes a shiftregister, a first latch, a second latch, and a timing signal controlcircuit, a timing signal output from the shift register is input intothe first latch via the timing signal control circuit, a digital videosignal is input into and held at the first latch by the timing signalinput into the first latch, the digital video signal held at the firstlatch is input into and held at the second latch by a latch signal, thedigital video signal input into and held at the second latch is inputinto the plurality of pixels, and the timing signal control circuitreduces the number of bits of the digital video signal to be input intoand held at the first latch by supplying to the first latch a constantfixed electrical potential instead of the timing signal output from theshift register for a constant period of time.

The above structure may be characterized in which the timing signalcontrol circuit includes a NAND and an inverter, wherein a timing signaland a selection signal are input into the NAND, and a signal output fromthe NAND is output from the timing signal control circuit via theinverter.

The above structure may be characterized in which the timing signalcontrol circuit includes a first analog switch, a second analog switch,and an inverter, wherein a selection signal is input via the inverterinto a second control input terminal of the first analog switch and afirst control input terminal of the second analog switch, the selectionsignal is input into a first control input terminal of the first analogswitch and a second control input terminal of the second analog switch,the timing signal is input into an input terminal of the first analogswitch, a fixed electrical potential is supplied to an input terminal ofthe second analog switch, and signals output from output terminals ofthe first analog switch and the second analog switch are output from thetiming signal control circuit.

The present invention provides a display device having a pixel portionincluding a plurality of pixels and a source signal line driver circuit,wherein the source signal line driver circuit includes a shift register,a first latch, a second latch, and a start pulse signal control circuit,a start pulse signal is input into the shift register via the startpulse signal control circuit to thereby output a timing signal from theshift register, a digital video signal is input into and held at thefirst latch by the timing signal, the digital video signal held at thefirst latch is input into and held at the second latch by a latchsignal, the digital video signal input into and held at the second latchis input into the plurality of pixels, and the start pulse signalcontrol circuit reduces the number of bits of the digital video signalto be input into and held at the first latch by supplying to the shiftregister a constant fixed electrical potential instead of the startpulse for a constant period of time.

The above structure may be characterized in which the start pulse signalcontrol circuit includes a NAND and an inverter, wherein a start pulsesignal and a selection signal are input into the NAND, and a signaloutput from the NAND is output from the start pulse signal controlcircuit via the inverter.

The above structure may be characterized in which the start pulse signalcontrol circuit includes a first analog switch, a second analog switch,and an inverter, wherein a selection signal is input via the inverterinto a second control input terminal of the first analog switch and afirst control input terminal of the second analog switch, the selectionsignal is input into a first control input terminal of the first analogswitch and a second control input terminal of the second analog switch,a start pulse signal is input into an input terminal of the first analogswitch, a fixed electrical potential is supplied to an input terminal ofthe second analog switch, and signals output from output terminals ofthe first analog switch and the second analog switch are output from thestart pulse signal control circuit.

The present invention provides a display device including a plurality ofpixels having a plurality of light emitting elements, and a monitoringlight emitting element, wherein the magnitude of a current to flowthrough the plurality of light emitting elements is maintained by meansof temperature characteristics of the monitoring light emitting element.

The present invention provides a display device including a pixelportion including a plurality of pixels, a power source line, a bufferamplifier, a monitoring light emitting element, and a constant currentsource, wherein each of the plurality of pixels includes a thin filmtransistor and a light emitting element, each of the monitoring lightemitting element and the light emitting element includes a firstelectrode, a second electrode, and an organic compound layer disposedbetween the first electrode and the second electrode, the firstelectrode of the monitoring light emitting element is connected to theconstant current source, the first electrode of the monitoring lightemitting element is also connected to a non-inverted input terminal ofthe buffer amplifier, and an output terminal of the buffer amplifier isconnected to the power source line, and an electrical potential of thepower source line is applied to the first electrode of the lightemitting element via the thin film transistor.

The present invention provides a display device including a pixelportion having a plurality of pixels, a power source line, a bufferamplifier, a monitoring light emitting element, a constant currentsource, and an adding circuit, wherein each of the plurality of pixelsincludes a thin film transistor and a light emitting element, each ofthe monitoring light emitting element and the light emitting elementincludes a first electrode, a second electrode, and an organic compoundlayer disposed between the first electrode and the second electrode, thefirst electrode of the monitoring light emitting element is connected tothe constant current source, the first electrode of the monitoring lightemitting element is also connected to a non-inverted input terminal ofthe buffer amplifier, an output terminal of the buffer amplifier isconnected to an input terminal of the adding circuit, an output terminalof the adding circuit is connected to the power source line, a constantpotential difference is always maintained between the input terminal andthe output terminal of the adding circuit, and an electrical potentialof the power source line is applied to the first electrode of the lightemitting element via the thin film transistor.

The present invention may be embodied in the form of a video camera, animage reproduction apparatus, a head mount display, a portabletelephone, or a portable information terminal employing theabove-mentioned display device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a light emitting device in accordance withthe present invention.

FIG. 2 is a block diagram of a light emitting device in accordance withthe present invention.

FIG. 3 is a block diagram of a light emitting device in accordance withthe present invention.

FIG. 4 is a block diagram of a light emitting device in accordance withthe present invention.

FIG. 5 is a diagram for illustrating connection of a monitoring lightemitting element of a light emitting device in accordance with thepresent invention.

FIG. 6 is a diagram for illustrating a pixel portion of a light emittingdevice in accordance with the present invention.

FIG. 7 is an expanded diagram of a pixel of a light emitting device inaccordance with the present invention.

FIG. 8 is a diagram for illustrating a driving method of a lightemitting device in accordance with the present invention.

FIG. 9 is a block diagram of a light emitting device in accordance withthe present invention.

FIG. 10 is a diagram for illustrating a pixel portion of a lightemitting device in accordance with the present invention.

FIG. 11 is an expanded diagram of a pixel of a light emitting device inaccordance with the present invention.

FIG. 12 is a diagram for illustrating a driving method of a lightemitting device in accordance with the present invention.

FIG. 13 is a circuit diagram of a source signal line driver circuit of alight emitting device in accordance with the present invention.

FIG. 14 is a partial plan view of a latch (A).

FIGS. 15A and 15B are circuit diagrams of a switching circuit.

FIG. 16 is an equivalent circuit diagram of an analog switch.

FIG. 17 is a circuit diagram of a source signal line driver circuit of alight emitting device in accordance with the present invention.

FIGS. 18A and 18B are circuit diagrams of a clock signal controlcircuit, a timing signal control circuit, and a start pulse signalcontrol circuit.

FIG. 19 is a circuit diagram of a source signal line driver circuit of alight emitting device in accordance with the present invention.

FIG. 20 is a circuit diagram of a source signal line driver circuit of alight emitting device in accordance with the present invention.

FIG. 21 is a diagram for illustrating connection of a monitoring lightemitting element of a light emitting device in accordance with thepresent invention.

FIG. 22 is a circuit diagram of an adding circuit.

FIGS. 23A through 23D are cross-sectional views for illustrating varioussteps of a fabricating method of a light emitting device.

FIGS. 24A through 24C are cross-sectional views for illustrating varioussteps of a fabricating method of a light emitting device.

FIGS. 25A through 25C are cross-sectional views for illustrating varioussteps of a fabricating method of a light emitting device.

FIGS. 26A and 26B are cross-sectional views for illustrating varioussteps of a fabricating method of a light emitting device.

FIGS. 27A and 27B are diagram for illustrating electronic apparatuseseach employing a light emitting device in accordance with the presentinvention.

FIGS. 28A through 28F are diagram for illustrating electronicapparatuses each employing a light emitting device in accordance withthe present invention.

FIGS. 29A and 29B show a connection of a light emitting element for amonitor of the light emitting device of the present invention, and acharacteristic in measurement values of brightness due to temperaturesof the light emitting element, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

A first structure in accordance with the present invention will be nowdescribed. FIG. 1 shows a block diagram of a light emitting devicehaving the first structure in accordance with the present invention.

Reference numeral 101 denotes a pixel portion in which a plurality ofpixels are provided in matrix. Reference numerals 102 and 103 denote asource signal line driver circuit and a gate signal line driver circuit,respectively.

The source signal line driver circuit 102 includes a shift register102-1, a latch (A) 102-2, a latch (B) 102-3, and a switching circuit102-4. It should be noted that the source signal line driver circuit ofthe present invention may further include a level shift, a buffer or thelike, in addition to the components mentioned above.

Although not illustrated, the gate signal line driver circuit 103includes a shift register and a buffer. In some cases, the gate signalline driver circuit 103 may further include a level shift in addition tothe shift register and the buffer. Gate electrodes of pixel TFTs in onerow are connected to one gate signal line, and therefore, all of thepixels TFTs in one row have to be simultaneously turned on. Accordingly,a buffer capable of allowing a large current to flow therethrough isused.

In the source signal line driver circuit 102, a clock signal (CLK) and astart pulse (SP) are input into the shift register 102-1. The shiftregister 102-1 sequentially generates a timing signal based on theseclock signal (CLK) and start pulse (SP) and supplies the generatedtiming signal sequentially to a circuit in the succeeding stage.

The timing signal to be output from the shift register 102-1 may besequentially supplied to the circuit in the succeeding stage through abuffer or the like (not illustrated). The timing signal from the shiftregister 102-1 is buffered and amplified by the buffer or the like.Since a number of circuits or elements are connected to a wiring towhich the timing signal is to be supplied, a large load capacitance(parasitic capacitance) exists. In order to prevent a rising edge or atrailing edge of the timing signal from being rounded due to such alarge load capacitance, the above-mentioned buffer is provided.

The timing signal output from the shift register 102-1 is supplied tothe latch (A) 102-2. The latch (A) 102-2 includes latches in multiplestages for processing an n-bit digital video signal. When the timingsignal is input to the latch (A) 102-2, it sequentially takes in then-bit digital video signal to be externally supplied to the sourcesignal line driver circuit 102 and held them therein.

When the digital video signal is taken in into the latch (A) 102-2, thedigital video signal may be sequentially input into the latches in themultiple stages included in the latch (A) 102-2. However, the presentinvention is not limited to such a structure. A so-called divisiondriving may be performed in which the latches in the multiple stagesincluded in the latch (A) 102-2 are divided into several groups and thedigital video signal is input every group in parallel andsimultaneously. In such a case, the number of the groups is referred toas the division number. For example, in the case where the latches aredivided into four groups, it can be said that the device is driventhrough the quarter-division scheme.

A time period required for completing all of the writing of the digitalvideo signal into the latches in all of the stages of the latch (A)102-2 is referred to as a line period. More specifically, the lineperiod refers to a time period measured from a timing when the writingof the digital video signal into the latch in the leftmost stage in thelatch (A) 102-2 starts until a timing when the writing of the digitalvideo signal into the latch in the rightmost stage ends. In practice,the line period may include a horizontal blanking period in addition tothe above-described line period.

After the one line period ends, latch signals are supplied to the latch(B) 102-3. At this moment, the digital video signal that has beenwritten into and held at the latch (A) 102-2 is simultaneously sent outto the latch (B) 102-3 to be written into latches in all of the stagesof the latch (B) 102-3 and held therein.

Further digital video signal to be externally supplied to the sourcesignal line driver circuit 102 is again sequentially written, based onthe timing signal from the shift register 102-1, into the latch (A)102-2 from which the previous digital video signal was sent out to thelatch (B) 102-3.

During the second turn of the one line period, the digital video signalthat has been written into and held at the latch (B) 102-3 issimultaneously sent out to the switching circuit 102-4. The switchingcircuit 102-4 causes the polarity of the digital video signal input fromthe latch (B) 102-2 to be inverted, or alternatively allows the polarityof those digital video signal not to be inverted, in accordance withshift signals, and outputs the resultant signals.

The digital video signal includes information of either “0” or “1”. Thedigital video signal corresponding to “0” is a signal having a Hipotential, while the digital video signal corresponding to “1” is asignal having a Lo potential, or vice versa. The inversion of thepolarity of the digital video signal means that the digital video signalhaving information of “0” is converted into that having information of“1”, while the digital video signal having information of “1” isconverted into that having information of “0”.

The shift signal is a signal to be used for selecting whether thepolarity of the digital video signal input from the latch (B) 102-2 isto be inverted or not. The polarity of the digital video signal isinverted by the shift signal when an average of a time period duringwhich all of the light emitting elements emit light in one frame periodbecomes longer than a half of a time period during which the lightemitting elements emit light in one frame period in the case where thetotally white is to be displayed in the pixel portion 101, therebyresulting in power consumption being reduced. On the other hand, when anaverage of the time period during which all of the light emittingelements emit light in one frame period becomes shorter than a half ofthe time period during which the light emitting elements emit light inone frame period in the case where the totally white is to be displayedin the pixel portion 101, the polarity of the digital video signal isnot allowed to be inverted in accordance with the shift signal, for thepurpose of suppressing the power consumption.

Whether the polarity of the digital video signal should be inverted ornot in accordance with the shift signal may be chosen by a user, oralternatively, may be automatically chosen based on an image to bedisplayed.

The digital video signal output from the switching circuit 102-4 isinput into the source signal lines.

On the other hand, in the gate signal line driver circuit 103, a gatesignal from a shift register (not illustrated) is input into a buffer(not illustrated), and further input into a corresponding gate signalline (also referred to as a scanning line).

In accordance with the gate signal input into the gate signal line, thedigital video signal input into the source signal lines is input intothe pixels.

In the above description, the source signal line driver circuit 102 andthe gate signal line driver circuit 103 may be provided on the samesubstrate as the pixel portion 101. Alternatively, those driver circuits102 and 103 may be provided on an IC chip and connected to the pixelportion 101 via some techniques such as FPC or TAB.

In accordance with the above-mentioned structure of the presentembodiment of the invention, in the case where the light emitting devicefor performing a digitally-driven time-divisional gradation displaydisplays a monochrome image, the contrast of an image can be invertedbased on natures of the image to be displayed in the pixel portion. Morespecifically, the contrast of the image to be displayed in the pixelportion is inverted when an average of a time period during which all ofthe light emitting elements emit light in one frame period becomeslonger than a half of a time period during which the light emittingelements emit light in one frame period in the case where the totallywhite is to be displayed in the pixel portion 101. On the other hand,when an average of the time period during which all of the lightemitting elements emit light in one frame period becomes shorter than ahalf of the time period during which the light emitting elements emitlight in one frame period in the case where the totally white is to bedisplayed in the pixel portion 101, it is desirable that the contrast ofthe image to be displayed in the pixel portion is not allowed to beinverted.

In the above description of the present embodiment, the switchingcircuit is included in the source signal line driver circuit.Alternatively, it is acceptable that the switching circuit is notincluded in the source signal line driver circuit.

In the present embodiment mode, only the situation where the digitalvideo signal is employed has been described. Alternatively, the presentinvention may be applied to the case where an analog video signal isused instead of the digital video signal.

Accordingly, the magnitude of a current to flow through the lightemitting element can be suppressed to some extent in accordance with thefirst structure of the present invention, thereby resulting in the powerconsumption of the resultant light emitting device being suppressed.

Embodiment Mode 2

A second structure in accordance with the present invention will be nowdescribed. FIG. 2 shows a block diagram of a light emitting devicehaving the second structure in accordance with the present invention. InFIG. 2, the same components as shown in FIG. 1 are designated with thesame reference numerals.

The light emitting device in the present embodiment mode employs a clocksignal control circuit 106 which allows a constant electrical potentialinstead of the clock signal (CLK) to be applied to the shift register102-1.

More specifically, the light emitting device is configured to input aconstant electrical potential (fixed electrical potential) into theshift register 102-1, instead of the clock signal, for a constant timeperiod by means of the clock signal control circuit 106. The abovestructure prevents the timing signal that is to be used for inputtingthe digital video signal at less significant bits in the range from thefirst bit to the m-th bit (m is any integer in the range from 1 to n)into the latch (A) 102-2 from being input into the latch (A) 102-2.Accordingly, only the digital video signal at more significant bits inthe range from the (m+1)-th bit to the n-th bit can be written into thelatch (A) 102-2.

The light emitting device in the present embodiment mode is differentfrom that shown in FIG. 1 in that the source signal line driver circuit102 does not include the switching circuit 102-4. Accordingly, thedigital video signal that has been written into and held at the latch(B) 102-3 is input into the source signal lines by means of the latchsignal to be input into the latch (B) 102-3.

In accordance with the present embodiment mode, in the light emittingdevice that performs a digitally-driven time-divisional gradationdisplay, the digital video signal input into the source signal linedriver circuit contained in the light emitting device is input into thepixel portion after the number of bits thereof is reduced. Morespecifically, the number of bits of the digital video signal to be inputinto the pixel portion is reduced by sequentially cutting off a bit ofthe digital video signal from the least significant bit.

In accordance with the above-described structure, the number of bits ofthe digital video signal to be input into the pixel portion is reduced,and therefore, the required number of writing the digital video signalinto the pixels by means of the source signal line driver circuit andthe gate signal line driver circuit can be reduced. Thus, the powerconsumption of the source signal line driver circuit and the gate signalline driver circuit can be suppressed, thereby resulting in the powerconsumption of the light emitting device being suppressed.

It should be noted that in the present embodiment mode, the clock signalcontrol circuit 106 may be provided on the same substrate as the pixelportion 101, or alternatively, may be formed in an IC chip.

Embodiment Mode 3

Another example of the second structure in accordance with the presentinvention, which is different from the one described in Embodiment Mode2, will be now described. FIG. 3 shows a block diagram of a lightemitting device having the second structure in accordance with thepresent invention. In FIG. 3, the same components as shown in FIG. 1 aredesignated with the same reference numerals.

The light emitting device in the present embodiment mode employs atiming signal control circuit 107 which allows a constant electricalpotential, instead of the timing signal output from the shift register102-1, to be applied to the latch (A) 102-2.

More specifically, the light emitting device is configured to input aconstant electrical potential (fixed electrical potential) into thelatch (A) 102-2, instead of the timing signal output from the shiftregister 102-1, for a constant time period by means of the timing signalcontrol circuit 107. The above structure prevents the timing signal thatis to be used for inputting the digital video signal at less significantbits in the range from the first bit to the m-th bit (m is any integerin the range from 1 to n) into the latch (A) 102-2 from being input intothe latch (A) 102-2. Accordingly, only the digital video signal at moresignificant bits in the range from the (m+1)-th bit to the n-th bit canbe written into the latch (A) 102-2.

It should be noted that in the present embodiment mode, the fixedelectrical potential is required to be at such a level that prevents thedigital video signal from being written into the latch (A) 102-2.

In accordance with the present embodiment mode, in the light emittingdevice that performs a digitally-driven time-divisional gradationdisplay, the digital video signal input into the source signal linedriver circuit contained in the light emitting device is input into thepixel portion after the number of bits thereof is reduced. Morespecifically, the number of bits of the digital video signal to be inputinto the pixel portion is reduced by sequentially cutting off the bit ofthe digital video signal from the least significant bit.

In accordance with the above-described structure, the number of bits ofthe digital video signal to be input into the pixel portion is reduced,and therefore, the required number of writing the digital video signalinto the pixels by means of the source signal line driver circuit andthe gate signal line driver circuit can be reduced. Thus, the powerconsumption of the source signal line driver circuit and the gate signalline driver circuit can be suppressed, thereby resulting in the powerconsumption of the light emitting device being suppressed.

It should be noted that in the present embodiment mode, the timingsignal control circuit 107 may be provided on the same substrate as thepixel portion 101, or alternatively, may be formed in an IC chip.

Embodiment Mode 4

Still another example of the second structure in accordance with thepresent invention, which is different from the ones described inEmbodiment Modes 2 and 3, will be now described. FIG. 4 shows a blockdiagram of a light emitting device having the second structure inaccordance with the present invention. In FIG. 4, the same components asshown in FIG. 1 are designated with the same reference numerals.

The light emitting device in the present embodiment mode employs a startpulse signal control circuit 108 which allows a constant electricalpotential, instead of the start pulse signal (SP), to be applied to theshift register 102-1.

More specifically, the light emitting device is configured to input aconstant electrical potential (fixed electrical potential) into theshift register 102-1, instead of the start pulse signal, for a constanttime period by means of the start pulse signal control circuit 108, sothat the timing signal that is to be used for inputting the digitalvideo signal at less significant bits in the range from the first bit tothe m-th bit (m is any integer in the range from 1 to n) into the latch(A) 102-2 is prevented from being input into the latch (A) 102-2.Accordingly, only the digital video signal at more significant bits inthe range from the (m+1)-th bit to the n-th bit can be written into thelatch (A) 102-2.

It should be noted that in the present embodiment mode, the fixedelectrical potential is required to be at such a level that prevents thetiming signal from being output from the shift register 102-1.

In accordance with the present embodiment mode, in the light emittingdevice that performs a digitally-driven time-divisional gradationdisplay, the digital video signal input into the source signal linedriver circuit contained in the light emitting device is input into thepixel portion after the number of bits thereof is reduced. Morespecifically, the number of bits of the digital video signal to be inputinto the pixel portion is reduced by sequentially cutting off the bit ofthe digital video signal from the least significant bit.

In accordance with the above-described structure, the number of bits ofthe digital video signal to be input into the pixel portion is reduced,and therefore, the required number of writing the digital video signalinto the pixels by means of the source signal line driver circuit andthe gate signal line driver circuit can be reduced. Thus, the powerconsumption of the source signal line driver circuit and the gate signalline driver circuit can be suppressed, thereby resulting in the powerconsumption of the light emitting device being suppressed.

It should be noted that in the present embodiment mode, the start pulsesignal control circuit 108 may be provided on the same substrate as thepixel portion 101, or alternatively, may be formed in an IC chip.

Embodiment Mode 5

A third structure in accordance with the present invention will be nowdescribed with reference to FIG. 5.

In FIG. 5, reference numeral 501 denotes a power source line. In thepresent specification, the power source line refers to a wiring to beused for applying a predetermined electrical potential to a pixelelectrode contained in a light emitting element in a pixel section bymeans of the digital video signal to be input into a source signal line.In the present specification, the electrical potential of the powersource line is referred to as the power source potential.

Reference numeral 502 denotes a buffer amplifier, reference numeral 503denotes a monitoring light emitting element, and reference numeral 504denotes a constant current source. One of electrodes of the monitoringlight emitting element 503 is connected to the constant current source504, so that a constant current always flows through the monitoringlight emitting element 503. When a temperature of an organic compoundlayer contained in the light emitting element changes, the magnitude ofthe current to flow through the monitoring light emitting element 503does not change, but rather, an electrical potential of the electrode ofthe monitoring light emitting element 503 connected to the constantcurrent source 504 changes.

On the other hand, the buffer amplifier 502 includes two input terminalsand one output terminal. One of the two input terminals is anon-inverted input terminal (+), while the other is an inverted inputterminal (−). An electrical potential at one of electrodes of themonitoring light emitting element 503 is supplied to the bufferamplifier 502.

The buffer amplifier 502 is a circuit for preventing an electricalpotential at a pixel electrode of the monitoring light emitting element503 connected to the constant current source 504 changes in accordancewith a load such as a wiring capacitance of the power source line 501 orthe like. Accordingly, the electrical potential provided to thenon-inverted input terminal of the buffer amplifier 502 is output froman output terminal to be supplied to the power source line as the powersource potential, without being changed in accordance with a load suchas a wiring capacitance of the power source line 501 or the like.

Accordingly, even when a temperature of the monitoring light emittingelement 503 or the organic compound layer of the light emitting elementin the pixel portion changes due to a change in an environmentaltemperature, the power source potential is changed so as to allow aconstant current to flow through the light emitting element. Thus, evenwhen the environmental temperature of the light emitting deviceincreases, power consumption of the light emitting device can beprevented from increasing.

In the present embodiment mode, the buffer amplifier 502, the monitoringlight emitting element 503, and the constant current source 504 may beprovided on the same substrate as the pixel portion, or alternatively,may be formed in an IC chip. Moreover, the monitoring light emittingelement 503 may be included in the pixel portion, or alternatively, maybe provided independently of the pixel portion.

The present invention can suppress power consumption of a light emittingdevice and an electronic apparatus employing the light emitting devicein accordance with the above-described first through third structures.It should be noted that only either one of the first through thirdstructures is required to be included in the present invention, althoughtwo or all of the first through third structures may be included.

The present invention can allow power consumption of the light emittingdevice to be suppressed by means of the above-described threestructures.

EMBODIMENTS

Several embodiments will be described below.

Embodiment 1

In the present embodiment, the structure of a pixel portion of a lightemitting device and its driving method in accordance with the presentinvention will be described.

FIG. 6 shows an expanded view of the pixel portion 301 of the lightemitting device in accordance with the present embodiment of theinvention. Source signal lines (S1 to Sx), power source lines (V1 toVx), and gate signal lines (G1 to Gy) are provided in the pixel portion301.

In the present embodiment, a pixel 304 refers to a region in which oneof the source signal lines (S1 to Sx), one of the power source lines (V1to Vx), and one of the gate signal lines (G1 to Gy) are provided. In thepixel portion 301, a plurality of the pixels 304 are arranged in matrix.

An expanded view of the pixel 304 is shown in FIG. 7. In FIG. 7,reference numeral 305 denotes a switching TFT. A gate electrode of theswitching TFT 305 is connected to the gate signal line G (G1 to Gx). Oneof a source region and a drain region of the switching TFT 305 isconnected to the source signal line S (S1 to Sx), while the otherthereof is connected to a gate electrode 306 of a current-controllingTFT 306 as well as to a capacitor 308 of each pixels.

The capacitor 308 is provided for holding a gate potential of thecurrent-controlling TFT 306 (an electrical potential difference betweenthe gate electrode and the source region) when the switching TFT 305 isin the non-selected condition (off condition). Although the capacitor308 is provided in the present embodiment, the present invention is notlimited to such a structure. The capacitor 308 may be omitted.

One of the source region and the drain region of the current-controllingTFT 306 is connected to the power source line V (V1 to Vx), while theother is connected to the light emitting element 307. The power sourceline V is connected to the capacitor 308.

The light emitting element 307 is composed of an anode, a cathode, andan organic compound layer provided between the anode and the cathode. Inthe case where the anode is connected to the source region or the drainregion of the current-controlling TFT 306, the anode functions as thepixel electrode and the cathode functions as a counter electrode. On theother hand, in the case where the cathode is connected to the sourceregion or the drain region of the current-controlling TFT 306, thecathode functions as the pixel electrode and the anode functions as thecounter electrode.

A counter potential is supplied to the counter electrode of the lightemitting element 307. In addition, the power source potential issupplied to the power source line V. The power source potential and thecounter potential are supplied to the light emitting device of thepresent invention by means of a power source provided by anexternally-attached IC chip or the like.

The switching TFT 305 and the current-controlling TFT 306 may be eitherof the n-channel type TFT or the p-channel type TFT. It should be noted,however, that in the case where the source region or the drain region ofthe current-controlling TFT 306 is connected to the anode of the lightemitting element 307, it is preferable that the current-controlling TFT306 is the p-channel type TFT. On the other hand, in the case where thesource region or the drain region of the current-controlling TFT 306 isconnected to the cathode of the light emitting element 307, it ispreferable that the current-controlling TFT 306 is the n-channel typeTFT.

The switching TFT 305 and the current-controlling TFT 306 may have amulti-gate structure such as a double gate structure, a triple gatestructure or the like, rather than a single gate structure.

Then, a driving method of the light emitting device of the presentinvention that includes the above-mentioned structure will be describedwith reference to FIG. 8.

First, the power source potential of the power source line becomes to beequal to an electrical potential of the counter electrode of the lightemitting element. Then, a gate signal is input from the gate signal linedriver circuit to the gate signal line G1. As a result, the switchingTFTs 305 of all of the pixels connected to the gate signal line G1(i.e., the pixels in the first row) are placed in the ON state.

Simultaneously, the digital video signal at the first bit is input fromthe source signal line driver circuit into the source signal lines (S1to Sx). The digital video signal is input to the gate electrode of thecurrent-controlling TFT 306 via the switching TFT 305.

Then, at the same time when the input of the gate signal into the G1 iscompleted, the similar gate signal is input into the next gate signalline G2. Thus, the switching TFTs 305 of all of the pixels connected tothe gate signal line G2 (i.e., the pixels in the second row) are placedin the ON state, thereby the digital video signal at the first bit isinput from the source signal lines (S1 to Sx) into the pixels in thesecond row.

Thereafter, the gate signal is sequentially input to all of the gatesignal lines (G1 to Gx). A time period required for selecting all of thegate signal lines (G1 to Gx) and inputting the digital video signal atthe first bit into the pixels in all of the rows is a writing periodTa1.

When the writing period Ta1 is completed, a light emitting period Tr1then starts. During the light emitting period Tr1, the power sourcepotential of the power source line comes to be at such a potential levelthat provides a potential difference with respect to the counterelectrode, so that the light emitting element can emit light when thepower source potential is supplied to the pixel electrode of the lightemitting element.

In the present embodiment, in the case where the digital video signalhas information of “0”, the current-controlling TFT 306 is in the OFFstate. Accordingly, the power source potential is not supplied to thepixel electrode of the light emitting element 307. As a result, thelight emitting element 307 included in the pixel to which the digitalvideo signal having information of “0” is supplied does not emit light.

On the other hand, in the case where the digital video signal hasinformation of “1”, the current-controlling TFT 306 is in the ON state.Accordingly, the power source potential is supplied to the pixelelectrode of the light emitting element 307. As a result, the lightemitting element 307 included in the pixel to which the digital videosignal having information of “1” is supplied emits light.

Thus, during the display period Tr1, the light emitting element 307 isplaced either in the emission state or the non-emission state, so thatall of the pixels perform a display operation. A time period duringwhich the pixel performs the display operation is referred to as adisplay period Tr. More specifically, the display period that startswhen the digital video signal at the first bit is input into the pixelis referred to as Tr1. In FIG. 8, only the display period of the pixelsin the first row is illustrated for the purpose of simplification of thedescription. Timings at which the respective display periods for all ofthe rows start are the same.

When the display period Tr1 is completed, a next writing period Ta2starts, and the power source potential of the power source line comes tobe equal to the electrical potential of the counter electrode of thelight emitting element. Similarly in the case of the writing period Ta1,all of the gate signal lines are sequentially selected, and the digitalvideo signal at the second bit is input into all of the pixels. A timeperiod required for completing the input of the digital video signal atthe second bit into the pixels in all of the rows is referred to as awriting period Ta2.

When the writing period Ta2 is completed, a display period Tr2 thenstarts, and the power source potential of the power source line comes tobe at such a potential level that provides a potential difference withrespect to the counter electrode, so that the light emitting element canemit light when the power source potential is supplied to the pixelelectrode of the light emitting element. Thus, all of the pixels performthe display operation.

The above-described operations are repeated until the digital videosignal at the n-th bit is input into the pixels, so that the writingperiod Ta and the display period Tr appear alternately. After all of thedisplay periods (Tr1 to Trn) are completed, one image can be displayed.In the driving method in accordance with the present invention, a timeperiod required for displaying one image is referred to as one frameperiod (F). After a certain one frame period (F) is completed, the nextone frame period is started. The writing period Ta1 again appears, andthe above-described operations are repeated.

In a normal light emitting device, it is preferable to provide 60 ormore frame periods per one second. If the number of images to bedisplayed in one second is smaller than 60, flickering of the image maybecome visually significant.

In the present embodiment, it is necessary that the sum of all of thewriting periods is shorter than one frame period and ratios among therespective display periods are set to satisfy the relationship of Tr1:Tr2: Tr3: . . . : Tr(n−1): Trn=2⁰: 2¹: 2²: . . . : 2^((n−2)): 2^((n−1)).Through the combination of the display periods, a desired gradationdisplay among the total of 2^(n) gradations can be realized.

The gradation to be displayed by a particular pixel in a certain oneframe period is determined by obtaining the sum of the display periodsduring which the light emitting element emits light in that frameperiod. For example, assuming that n=8 and the brightness obtained whenthe pixel emits light in all of the display periods is expressed as100%, the brightness of 1% can be realized in the case where the pixelemits light in Tr1 and Tr2, while the brightness of 60% can be realizedwhen Tr3, Tr5, and Tr8 are selected.

The display periods Tr1 through Trn may appear on any order. Forexample, during one frame period, the display periods may be controlledto appear in such an order that Tr1 is followed by Tr3, Tr5, Tr2, . . ..

Although the level of the power source potential at the power sourceline is changed by means of the writing period and the display period inthe above description of the present embodiment, the present inventionis not limited thereto. A potential difference which allows the lightemitting element to emit light when the power source potential issupplied to the pixel electrode of the light emitting element may becontrolled to always exist between the power source potential and theelectrical potential at the counter electrode. In such a case, the lightemitting element can emit light even in the writing period. Accordingly,the gradation to be displayed by a particular pixel in a certain frameperiod is determined by the sum of the writing periods and the displayperiods during which the light emitting element emits light in thatframe period. In this case, it is necessary that the sum of the writingperiods and the display periods corresponding to the digital videosignal at the respective bits is set to satisfy the relationship of(Ta1+Tr1): (Ta2+Tr2): (Ta3+Tr3): . . . : (Ta(n−1)+Tr(n−1)):(Tan+Trn)=2⁰: 2¹: 2²: . . . : 2^((n−2)): 2^((n−1)).

Embodiment 2

In the present embodiment, another example of the structure of a pixelportion of a light emitting device and its driving method in accordancewith the present invention, different from those in the Embodiment 1,will be described.

FIG. 9 shows an exemplary block diagram of a light emitting device inthe present embodiment. The light emitting device in FIG. 9 includes apixel portion 901 by TFT formed on the substrate and includes a sourcesignal side driver circuit 902, a writing gate signal side drivercircuit (first gate signal line driver circuit) 903 a, and an erasuregate signal line driver circuit (second gate signal line driver circuit)903 b, each provided in the periphery of the pixel portion. Although thelight emitting device with one source signal side driver circuit isdescribed in the present embodiment, two of the source signal sidedriver circuits may be provided.

The source signal side driver circuit 902 has at least one of the firstthrough third structures of the present invention described previously.

In the present embodiment, the source signal line driver circuit 902 andthe writing gate signal side driver circuit 903 a and the erasure gatesignal line driver circuit 903 b may be provided on the same substrateas the pixel portion 901, or alternatively, may be formed on an IC chipand connected to the pixel portion 901 via a certain connector such asFPC, TAB or the like.

An expanded view of the pixel portion 901 is shown in FIG. 10. In FIG.10, source signal lines (S1 to Sx), power source lines (V1 to Vx), andwriting gate signal lines (first gate signal lines) (Ga1 to Gay), anderasure gate signal lines (second gate signal lines) (Ge1 to Gey) areprovided in the pixel portion 901.

The pixel 904 refers to a region in which one of the source signal lines(S1 to Sx), one of the power source lines (V1 to Vx), one of the writinggate signal lines (Ga1 to Gay), and one of the erasure gate signal lines(Ge1 to Gey) are provided. In the pixel portion 901, a plurality of thepixels 904 are arranged in matrix.

An expanded view of the pixel 904 is shown in FIG. 11. In FIG. 11,reference numeral 907 denotes the switching TFT. A gate electrode of theswitching TFT 907 is connected to the gate signal line Ga (Ga1 to Gay).One of a source region and a drain region of the switching TFT 907 isconnected to the source signal line S (S1 to Sx), while the otherthereof is connected to a gate electrode of a current-controlling TFT908 as well as to a capacitor 912 and source and drain regions of anerasure TFT 909 included in each pixels.

The capacitor 912 is provided for holding a gate potential of thecurrent-controlling TFT 908 when the switching TFT 907 is in thenon-selected condition (off condition). Although the capacitor 912 isprovided in the present embodiment, the present invention is not limitedto such a structure. The capacitor 912 may be omitted.

One of the source region and the drain region of the current-controllingTFT 908 is connected to the power source line V (V1 to Vx), while theother thereof is connected to the light emitting element 910. The powersource line V is connected to the capacitor 912.

One of the source region and the drain region of the erasure TFT 909that is not connected to the source or drain region of the switching TFT907 is connected to the power source line V. A gate electrode of theerasure TFT 909 is connected to the erasure gate signal line Ge.

The light emitting element 910 is composed of an anode, a cathode, andan organic compound layer provided between the anode and the cathode. Inthe case where the anode is connected to the source region or the drainregion of the current-controlling TFT 908, the anode functions as thepixel electrode and the cathode functions as a counter electrode. On theother hand, in the case where the cathode is connected to the sourceregion or the drain region of the current-controlling TFT 908, thecathode functions as the pixel electrode and the anode functions as thecounter electrode.

A counter potential is supplied to the counter electrode 911 of thelight emitting element 910. In addition, the power source potential issupplied to the power source line V. A potential difference between thecounter potential and the power source potential is always maintained atsuch a level that causes the light emitting element to emit light whenthe power source potential is applied to the pixel electrode. The powersource potential and the counter potential are supplied to the lightemitting device of the present invention by means of a power sourceprovided by an externally-attached IC chip or the like.

In the typical light emitting device to date, when the amount of lightemission per unit light emission area of a pixel is 200 cd/m², a currentof several mA/cm² per unit area of a pixel portion is required to flow.Thus, especially with a larger display area, it becomes difficult tocontrol the magnitude of the electrical potential to be supplied fromthe power source provided in the IC chip by means of a switch. In thepresent embodiment, the power source potential and the counter potentialare always maintained to be constant, and therefore, the magnitude ofthe electrical potential to be supplied from the power source providedin the IC chip is not required to be controlled by means of a switch.Accordingly, the present embodiment is useful for realizing a panelhaving a larger display size.

The switching TFT 907, the current-controlling TFT 908, and the erasureTFT 909 may be either of the n-channel type TFT or the p-channel typeTFT. It should be noted, however, that in the case where the sourceregion or the drain region of the current-controlling TFT 908 isconnected to the anode of the light emitting element 910, it ispreferable that the current-controlling TFT 908 is the p-channel typeTFT. On the other hand, in the case where the source region or the drainregion of the current-controlling TFT 908 is connected to the cathode ofthe light emitting element 910, it is preferable that thecurrent-controlling TFT 908 is the n-channel type TFT.

The switching TFT 907, the current-controlling TFT 908, and the erasureTFT 909 may have a multi-gate structure such as a double gate structure,a triple gate structure or the like, rather than a single gatestructure.

Then, a driving method of the light emitting device of the presentinvention that includes the above-mentioned structure will be describedwith reference to FIG. 12.

First, a writing gate signal is input from the writing gate signal linedriver circuit 903 a to the writing gate signal line Ga1, and thus, theswitching TFTs 907 of all of the pixels connected to the writing gatesignal line Ga1 (i.e., the pixels in the first row) are placed in the ONstate. In the present specification, when all of the TFTs having a gateelectrode connected to a certain signal line, the signal line isreferred to as being selected. Thus, in the above-described case, thewriting gate signal line Ga1 is being selected.

Simultaneously, the digital video signal at the first bit is input fromthe source signal line driver circuit 902 into the source signal lines(S1 to Sx). More specifically, the digital video signal is input to thegate electrode of the current-controlling TFT 908 via the switching TFT907.

In the present embodiment, in the case where the digital video signalhas information of “0”, the current-controlling TFT 908 is in the OFFstate. Accordingly, the power source potential is not supplied to thepixel electrode of the light emitting element 910. As a result, thelight emitting element 910 included in the pixel to which the digitalvideo signal having information of “0” is supplied does not emit light.

On the other hand, in the case where the digital video signal hasinformation of “1”, the current-controlling TFT 908 is in the ON state.Accordingly, the power source potential is supplied to the pixelelectrode of the light emitting element 910. As a result, the lightemitting element 910 included in the pixel to which the digital videosignal having information of “1” is supplied emits light.

Thus, at the same time when the digital video signal is input into thepixels in the first row, the light emitting element is placed either inthe emission state or the non-emission state, so that all of the pixelsin the first row perform a display operation. A time period during whichthe pixel performs the display operation is referred to as a displayperiod Tr. More specifically, the display period that starts when thedigital video signal at the first bit is input into the pixel isreferred to as Tr1. In FIG. 12, only the display period of the pixels inthe first row is illustrated for the purpose of simplification of thedescription. Timings at which the display periods for the respectiverows start are offset from each other by a certain time difference.

Then, at the same time when the selection of the Ga1 is completed, thenext writing gate signal line Ga2 is selected by the writing gatesignal. Thus, the switching TFTs 907 of all of the pixels connected tothe writing gate signal line Ga2 are placed in the ON state, thereby thedigital video signal at the first bit is input from the source signallines (S1 to Sx) into the pixels in the second row.

Thereafter, all of the writing gate signal lines (Ga1 to Gax) aresequentially selected. A time period required for selecting all of thewriting gate signal lines (Ga1 to Gax) and inputting the digital videosignal at the first bit into the pixels in all of the rows is a writingperiod Ta1.

On the other hand, before the digital video signal at the first bit isinput into the pixels in all of the rows, in other words, before thewriting period Ta1 is completed, the erasure gate signal line Ge1 isselected by an erasure gate signal to be input from the erasure gatesignal line driver circuit 903 b, in parallel to the input of thedigital video signal of the first bit into the pixels.

When the erasure gate signal line Ge1 is selected, the erasure TFTs 909in all of the pixels connected to the erasure gate signal line Ge1 (thepixels in the first row) are placed in the ON state. Thus, the powersource potential of the power source line (V1 to Vx) is supplied to thegate electrodes of the current-controlling TFTs 908 in the pixels in thefirst row.

When the power source potential is supplied to the gate electrodes ofthe current-controlling TFTs 908, the current-controlling TFTs 908 areplaced in the OFF state. Thus, the power source potential is notsupplied to the pixel electrodes of the light emitting elements 910, sothat all of the light emitting elements included in the pixels in thefirst row are placed in the non-emission state. Thus, the pixels in thefirst row do not performs a display operation. In other words, thedigital video signal that has been held at the gate electrode of thecurrent-controlling TFT since the selection of the writing gate signalline Ga1 is erased when the power source potential is supplied to thegate electrode of the current-controlling TFT. Thus, the pixels in thefirst row do not performs a display operation.

A time period during which the pixel does not perform a displayoperation is referred to as a non-display period Td. The display periodTr1 for the pixels in the first row is completed simultaneously when theerasure gate signal is input into the erasure gate signal line Ge1, andthe non-display period Td1 starts.

In FIG. 12, only the non-display period for the pixels in the first rowis particularly illustrated for the purpose of simplification of thedescription. Timings at which the non-display periods for the respectiverows start are offset from each other by a certain time difference.

Then, at the same time when the selection of the Ge1 is completed, thenext erasure gate signal line Ge2 is selected by the erasure gatesignal. Thus, the erasure TFTs 909 of all of the pixels connected to theerasure gate signal line Ge2 (the pixels in the second row) are placedin the ON state. The power source potential of the power source lines(V1 to Vx) is supplied to the gate electrode of the current-controllingTFT 908 via the erasure TFT 909. When the power source potential issupplied to the gate electrode of the current-controlling TFT 908, thecurrent-controlling TFT 908 is placed in the OFF state. Thus, the powersource potential is not supplied to the pixel electrode of the lightemitting element 910, so that all of the light emitting elementsincluded in the pixels in the second row are placed in the non-emissionstate. Thus, the pixels in the second row do not performs a displayoperation and are placed in the non-display state.

Then, all of the erasure gate signal lines are sequentially selected bythe erasure gate signal. A time period required for selecting all of theerasure gate signal lines (Ga1 to Gax) and erasing the digital videosignal at the first bit held in the pixels in all of the rows isreferred to as the erasure period Te1.

On the other hand, before the digital video signal at the first bit heldin the pixels in all of the rows is erased, in other words, before theerasure period Te1 is completed, the writing gate signal line Ga1 isagain selected, in parallel to the erasure of the digital video signalat the first bit in the pixels. Thus, the pixels in the first row againperform the display operation. The non-display period Td1 is thuscompleted, and a display period Tr2 starts.

Thereafter, all of the writing gate signal lines are sequentiallyselected in the similar manner as described before, and the digitalvideo signal at the second bit is input into all of the pixels. A timeperiod required for inputting the digital video signal at the second bitinto the pixels in all of the rows is referred to as a writing periodTa2.

On the other hand, before the digital video signal at the second bit isinput into the pixels in all of the rows, in other words, before thewriting period Ta2 is completed, the erasure gate signal line Ge2 isselected in parallel to the input of the digital video signal at thesecond bit into the pixels. Thus, all of the light emitting elementsincluded in the pixels in the first row are placed in the non-emissionstate. Thus, the pixels in the first row do not perform a displayoperation. The display period Tr2 for the pixels in the first row isthen completed, and the non-display period Td2 starts.

Thereafter, all of the erasure gate signal lines are sequentiallyselected. A time period required for selecting all of the erasure gatesignal lines (Ga1 to Gax) and erasing the digital video signal at thesecond bit held at the pixels in all of the rows is an erasure periodTe2.

The above-described operations are repeated until the digital videosignal at the m-th bit is input into the pixels, so that the displayperiod Tr and the non-display period Td appear alternately. The displayperiod Tr1 is a period from the start of the writing period Ta1 untilthe erasure period Te1 starts. The non-display period Td1 is a periodfrom the start of the erasure period Te1 until the display period Tr2starts. Similarly as the display period Tr1 and the non-display periodTd1, lengths of the display periods Tr2, Tr3, . . . , Tr(m−1) and thenon-display periods Td2, Td3, . . . , Td(m−1) are defined by means ofthe writing periods Ta1, Ta2, . . . , Tam and the erasure periods Te1,Te2, . . . , Te(m−1).

After the digital video signal at the m-th bit has been input into thepixels in the first row, the erasure gate signal line Ge1 is notselected. For the purpose of simplification of the description, the casewith m=n−2 is described as an example in the present embodiment.However, the present invention is not limited to such a case. In thepresent invention, as the value of m, any number in the range from 2 ton can be arbitrarily selected.

When the digital video signal at the (n−2)-th bit is input into thepixels in the first row, the pixels in the first row are placed in thedisplay period Tr(n−2) to perform a display operation. Until the nextdigital video signal at the next bit is input, the digital video signalat the (n−2)-th bit is held at the pixels.

When the digital video signal at the (n−1)-th bit is input into thepixels in the first row, the digital video signal at the (n−2)-th bitheld at the pixels are rewritten into the digital video signal at the(n−1)-th. The pixels in the first bit are then placed in the displayperiod Tr(n−1) to perform a display operation. Until the next digitalvideo signal at the next bit is input, the digital video signal at the(n−2)-th bit is held at the pixels.

The above-described operations are repeated until the digital videosignal at the n-th bit is input into the pixels. The display periodTr(n−2) is a period from the start of the writing period Ta(n−2) untilthe writing period Ta(n−1) starts. Similarly as the display periodTr(n−2), lengths of the display periods Tr(n−1) and Trn are defined bymeans of the writing period Ta.

In the present embodiment, it is necessary that the sum of all of thewriting periods is shorter than one frame period and ratios among therespective display periods are set to satisfy the relationship of Tr1:Tr2: Tr3: . . . : Tr(n−1): Trn=2⁰: 2¹: 2²: . . . : 2^((n−2)): 2^((n−1)).Through the combination of the display periods, a desired gradationdisplay among the total of 2^(n) gradations can be realized.

After all of the display periods (Tr1 through Trn) are completed, oneimage can be displayed. In the driving method in accordance with thepresent invention, a time period required for displaying one image isreferred to as one frame period (F).

After a certain one frame period (F) is completed, the digital videosignal at the first bit is again input into the pixels, so that thepixels in the first row are again placed in the display period Tr1. Theabove-mentioned operations are again repeated.

In a normal light emitting device, it is preferable to provide 60 ormore frame periods per one second. If the number of images to bedisplayed in one second is smaller than 60, flickering of the image maybecome visually significant.

The gradation to be displayed by a particular pixel in a certain oneframe period is determined by obtaining the sum of the display periodsduring which the light emitting element emits light in that frameperiod. For example, assuming that n=8 and the brightness obtained whenthe pixel emits light in all of the display periods is expressed as100%, the brightness of 1% can be realized in the case where the pixelemits light in Tr1 and Tr2, while the brightness of 60% can be realizedwhen Tr3, Tr5, and Tr8 are selected.

The writing period Tam during which the digital video signal at the m-thbit is input into the pixels is required to be shorter than the displayperiod Trm. Accordingly, the number of bits m is required to be such anumber from 1 to n that can allow the writing period Tam to be shorterthan the display period Trm.

The display periods (Tr1 through Trn) may appear on any order. Forexample, during one frame period, the display periods may be controlledto appear in such an order that Tr1 is followed by Tr4, Tr3, Tr2, . . .. It should be noted, however, that the appearing order of the displayperiods is preferably set so as not to cause the erasure periods (Te1through Ten) to be overlapped with each other.

In the present embodiment, the display period Tr and the writing periodTa are partially overlapped with each other. In other words, the pixelscan perform a display operation even during the writing period.Accordingly, a ratio of the sum of the display periods within one frameperiod (duty ratio) is not determined only by the length of the writingperiod.

Embodiment 3

In the present embodiment, the structure of the source signal linedriver circuit contained in the light emitting device as described inEmbodiment 1 will be described in detail. FIG. 13 shows a circuitdiagram of the source signal line driver circuit in the presentembodiment. In FIG. 13, the same components as those shown in FIG. 1 aredesignated with the same reference numerals.

Reference numeral 102-1 denotes a shift register, to which a clocksignal (CLK), a signal (CLKB) obtained by inverting the polarity of theclock signal, a start pulse signal (SP), a bi-direction shift signal(SL/R) are input through the illustrated wirings, respectively.

Reference numerals 102-2 and 102-3 denote a latch (A) and a latch (B),respectively. In the present embodiment, a combination of the latches(A) 102-2 and a combination of the latches (B) 102-3 corresponds to foursource signal lines. However, the number of source signal lines to whicha combination of the latches (A) 102-2 and a combination of the latches(B) 102-3 corresponds is not limited to the above number in the presentembodiment. In addition, although a level shift for changing a width ofa voltage amplitude of a signal is not provided in the presentembodiment, such a level shift may be appropriately provided by adesigner.

The digital video signal (DV) to be supplied externally to the sourcesignal line driver circuit is input into the latch (A) 102-2 through theillustrated wirings. A latch signal S_LAT and a signal S_LATb obtainedby inverting the polarity of the S_LAT are respectively input into thelatch (B) 102-3 through the illustrated wirings.

The structure of the latch (A) 102-2 will be described in detail withreference to a portion 801 of the latch (A) 102-2. The portion 801 ofthe latch (A) 102-2 includes two clocked inverters and two inverters.

A plan view of the portion 801 of the latch (A) 102-2 is shown in FIG.14. Reference numerals 831 a and 831 b respectively denote active layersof TFTs that constitute one of the inverters included in the portion 801of the latch (A) 102-2. Reference numeral 836 denotes a common gateelectrode of the TFTs that constitute the particular one of theinverters. Reference numerals 832 a and 832 b respectively denote activelayers of TFTs that constitute the other one of the inverters includedin the portion 801 of the latch (A) 102-2. Reference numerals 837 a and837 b denote gate electrodes formed over the active layers 832 a and 832b, respectively. The gate electrodes 837 a and 837 b are electricallyconnected to each other.

Reference numerals 833 a and 833 b respectively denote active layers ofTFTs that constitute one of the clocked inverters included in theportion 801 of the latch (A) 102-2. Gate electrodes 838 a and 838 b areprovided over the active layer 833 a to form a double gate structure.Similarly, gate electrodes 838 b and 839 are provided over the activelayer 833 b to form a double gate structure.

Reference numerals 834 a and 834 b respectively denote active layers ofTFTs that constitute the other one of the clocked inverters included inthe portion 801 of the latch (A) 102-2. Gate electrodes 839 and 840 areprovided over the active layer 834 a to form a double gate structure.Similarly, gate electrodes 840 and 841 are provided over the activelayer 834 b to form a double gate structure.

Reference numeral 102-4 denotes the switching circuit. FIGS. 15A and 15Bshows circuit diagrams of the switching circuit in accordance with thepresent embodiment.

The switching circuit 102-4 of the present embodiment as shown in FIG.15A includes an inverter 851, a first analog switch 852, and a secondanalog switch 853. A shift signal SS and a signal SSB obtained byinverting the polarity of the shift signal SS are input through theillustrated wirings.

Equivalent circuit diagrams of the first analog switch 852 and thesecond analog switch 853 are shown in FIG. 16. Each of the first analogswitch 852 and the second analog switch 853 includes an n-channel typeTFT and a p-channel type TFT. A signal to be input from an inputterminal (IN) is sampled by means of a signal to be input from a firstcontrol input terminal (Vin) or a second control input terminal (Vinb),and the resultant signals is output from an output terminal (OUT).

The digital video signal from the latch (B) 102-3 is input via theinverter 851 into the first analog switch 852 through the input terminal(IN). Simultaneously, the digital video signal from the latch (B) 102-3is input into the second analog switch 853 through the input terminal(IN).

The shift signal SS and the signal SSB obtained by inverting thepolarity of the shift signal SS are input into the first analog switch852 and the second analog switch 853, respectively, through the firstcontrol input terminal (Vin) or the second control input terminal(Vinb). The digital video signal is sampled by this shift signal SS, andthe sampled digital video signal is output from output terminals (OUT)of the first analog switch 852 and the second analog switch 853.

The digital video signal input into the switching circuit 102-4 isoutput therefrom after the polarity thereof is inverted, oralternatively, without having the polarity inverted. The shift signal SSdetermines whether the polarity of the digital video signal is to beinverted or not in the switching circuit 102-4.

The switching circuit 102-4 as shown in FIG. 15B includes an inverter861, a first NAND 862, a second NAND 863, and NOR 864. A shift signal SSand a signal SSB obtained by inverting the polarity of the shift signalSS are input through the illustrated wirings.

The digital video signal from the latch (B) 102-3 is provided throughthe inverter 861. Simultaneously, the signal SSB obtained by invertingthe polarity of the shift signal SS is input into the first NAND 862.

Simultaneously with the input of the digital video signal into the firstNAND 862 through the inverter 861, the digital video signal is alsoinput into the second NAND 863. Simultaneously, the shift signal SS isalso input into the second NAND 863.

Signals output from the first NAND 862 and the second NAND 863 aresimultaneously input into the NOR 864. A signal output from the NOR 864is input into the source signal line.

The digital video signal input into the switching circuit 102-4 isoutput therefrom after the polarity thereof is inverted, oralternatively, without having the polarity inverted. The shift signal SSdetermines whether the polarity of the digital video signal is to beinverted or not in the switching circuit 102-4.

The structure of the switching circuit is not limited to those shown inFIGS. 15A and 15B. The switching circuit may have any appropriatestructure as long as it can allow the digital video signal input theretoto be output therefrom with either the inverted polarity or thenon-inverted polarity.

The present embodiment can be freely combined with Embodiments 1 or 2.

Embodiment 4

In the present embodiment, the structure of the source signal linedriver circuit contained in the light emitting device as described inEmbodiment 2 will be described in detail. FIG. 17 shows a circuitdiagram of the source signal line driver circuit in the presentembodiment. In FIG. 17, the same components as those shown in FIG. 1 aredesignated with the same reference numerals.

Reference numeral 102-1 denotes a shift register, to which a clocksignal (CLK), a signal (CLKB) obtained by inverting the polarity of theclock signal, a start pulse signal (SP), a bi-direction shift signal(SL/R) are input through the illustrated wirings, respectively.

Reference numerals 102-2 and 102-3 denote a latch (A) and a latch (B),respectively. In the present embodiment, a combination of the latches(A) 102-2 and a combination of the latches (B) 102-3 correspond to foursource signal lines. However, the number of source signal lines to whicha combination of the latches (A) 102-2 and a combination of the latches(B) 102-3 correspond is not limited to the above number in the presentembodiment. In addition, although a level shift for changing a width ofa voltage amplitude of a signal is not provided in the presentembodiment, such a level shift may be appropriately provided by adesigner.

The digital video signal (DV) to be supplied externally to the sourcesignal line driver circuit is input into the latch (A) 102-2 through theillustrated wirings. A latch signal S_LAT and a signal S_LATb obtainedby inverting the polarity of the S_LAT are respectively input into thelatch (B) 102-3 through the illustrated wirings.

The detailed structure of the latch (A) 102-2 is the same as that shownin FIG. 14. Accordingly, the description thereof is omitted here.

Reference numeral 106 denotes the clock signal control circuit, that cansupply a constant electrical potential (fixed electrical potential),instead of the clock signal (CLK), to the shift register 102-1 for aconstant time period.

More specifically, the constant electrical potential (fixed electricalpotential) instead of the clock signal is input into the shift register102-1 for a constant time period by means of the clock signal controlcircuit 106, so that the timing signal that is to be used for inputtingthe digital video signal at less significant bits in the range from thefirst bit to the m-th bit into the latch (A) 102-2 is prevented frombeing input into the latch (A) 102-2. Accordingly, among the digitalvideo signal n input from external source signal line driver circuit,only the digital video signal at more significant bits in the range fromthe (m+1)-th bit to the n-th bit can be written into the latch (A)102-2.

FIGS. 18A and 18B show detailed circuit diagrams of the clock signalcontrol circuit 106 in accordance with the present embodiment.

The clock signal control circuit 106 of the present embodiment as shownin FIG. 18A includes a NAND 1801 and an inverter 1802. A selectionsignal is input through the illustrated wiring.

The clock signal to be input externally to the source signal line drivercircuit is input into the NAND 1801 through an input terminal (IN).Simultaneously, the selection signal is also input into the NAND 1801. Asignal output from the NAND 1801 is provided at an output terminal (OUT)after the polarity thereof is inverted by the inverter 1802 to be inputinto the shift register 102-1.

The selection signal determines whether the clock signal is to be inputinto the shift register 102-1, or the constant electrical potential(fixed electrical potential) is to be instead supplied thereto.

The clock signal control circuit 106 of the present embodiment as shownin FIG. 18B includes a first analog switch 1811, a second analog switch1812, and an inverter 1813. A selection signal is input through theillustrated wiring.

Equivalent circuit diagrams of the first analog switch 1811 and thesecond analog switch 1812 are the same as that shown in FIG. 16. Each ofthe first analog switch 1811 and the second analog switch 1812 includesan n-channel type TFT and a p-channel type TFT. A signal to be inputfrom an input terminal (IN) is sampled by means of a signal to be inputfrom a first control input terminal (Vin) or a second control inputterminal (Vinb), to be then output from an output terminal (OUT).

The selection signal is input into the first analog switch 1811 and thesecond analog switch 1812 through the first control input terminal(Vin). Simultaneously, the selection signal is also input into the firstanalog switch 1811 and the second analog switch 1812 through the secondcontrol input terminal (Vinb) after the polarity thereof is inverted.Further simultaneously, the clock signal to be supplied externally tothe source signal line driver circuit is input into the first analogswitch 1811 through the input terminal (IN). The second analog switch1812 is supplied with the constant electrical potential (fixedelectrical potential) through the input terminal (IN).

Signals respectively output from output terminals (OUT) of the firstanalog switch 1811 and the second analog switch 1812 are both outputfrom an output terminal of the clock signal control circuit 106.

The selection signal determines whether the clock signal is to be inputinto the shift register 102-1, or the constant electrical potential(fixed electrical potential) is to be instead supplied thereto.

The structure of the clock signal control circuit is not limited tothose shown in FIGS. 18A and 18B.

The present embodiment can be freely combined with Embodiments 1 through3.

Embodiment 5

In the present embodiment, the structure of the source signal linedriver circuit contained in the light emitting device as described inEmbodiment 3 will be described in detail. FIG. 19 shows a circuitdiagram of the source signal line driver circuit in the presentembodiment. In FIG. 19, the same components as those shown in FIG. 1 aredesignated with the same reference numerals.

Reference numeral 102-1 denotes a shift register, to which a clocksignal (CLK), a signal (CLKB) obtained by inverting the polarity of theclock signal, a start pulse signal (SP), a bi-direction shift signal(SL/R) are input through the illustrated wirings, respectively.

Reference numerals 102-2 and 102-3 denote a latch (A) and a latch (B),respectively. In the present embodiment, a combination of the latches(A) 102-2 and a combination of the latches (B) 102-3 correspond to foursource signal lines. However, the number of source signal lines to whicha combination of the latches (A) 102-2 and a combination of the latches(B) 102-3 correspond is not limited to the above number in the presentembodiment. In addition, although a level shift for changing a width ofa voltage amplitude of a signal is not provided in the presentembodiment, such a level shift may be appropriately provided by adesigner.

The digital video signal (DV) to be supplied externally to the sourcesignal line driver circuit is input into the latch (A) 102-2 through theillustrated wirings. A latch signal S_LAT and a signal S_LATb obtainedby inverting the polarity of the S_LAT are respectively input into thelatch (B) 102-3 through the illustrated wirings.

The detailed structure of the latch (A) 102-2 is the same as that shownin FIG. 14. Accordingly, the description thereof is omitted here.

Reference numeral 107 denotes the timing signal control circuit, thatcan supply a constant electrical potential (fixed electrical potential),instead of the timing signal, to the latch (A) 102-2 for a constant timeperiod.

More specifically, the constant electrical potential (fixed electricalpotential) instead of the timing signal is input into the shift register102-2 for a constant time period by means of the timing signal controlcircuit 107, so that the timing signal that is to be used for inputtingthe digital video signal at less significant bits in the range from thefirst bit to the m-th bit into the latch (A) 102-2 is prevented frombeing input into the latch (A) 102-2. Accordingly, only the digitalvideo signal at more significant bits in the range from the (m+1)-th bitto the n-th bit can be written into the latch (A) 102-2.

A structure of the timing signal control circuit 107 in the presentembodiment is the same as those shown in FIGS. 18A and 18B. Accordingly,the detailed description about the structure of the timing signalcontrol circuit 107 should be found in Embodiment 4. It should be noted,however, that in the present embodiment, the timing signal from theshift register 102-1 is input into the input terminal (IN) in thecircuits as shown in FIGS. 18A and 18B. A signal output from an outputterminal (OUT) in the circuits as shown in FIGS. 18A and 18B is inputinto the latch (A) 102-2. The selection signal determines whether thetiming signal is to be input into the latch (A) 102-2, or the constantelectrical potential (fixed electrical potential) is to be insteadsupplied thereto.

The structure of the timing signal control circuit is not limited tothose shown in FIGS. 18A and 18B.

The present embodiment can be freely combined with Embodiments 1 through3.

Embodiment 6

In the present embodiment, the structure of the source signal linedriver circuit contained in the light emitting device as described inEmbodiment 4 will be described in detail. FIG. 20 shows a circuitdiagram of the source signal line driver circuit in the presentembodiment. In FIG. 20, the same components as those shown in FIG. 1 aredesignated with the same reference numerals.

Reference numeral 102-1 denotes a shift register, to which a clocksignal (CLK), a signal (CLKB) obtained by inverting the polarity of theclock signal, a start pulse signal (SP), a bi-direction shift signal(SL/R) are input through the illustrated wirings, respectively.

Reference numerals 102-2 and 102-3 denote a latch (A) and a latch (B),respectively. In the present embodiment, a combination of the latches(A) 102-2 and a combination of the latches (B) 102-3 correspond to foursource signal lines. However, the number of source signal lines to whicha combination of the latches (A) 102-2 and a combination of the latches(B) 102-3 correspond is not limited to the above number in the presentembodiment. In addition, although a level shift for changing a width ofa voltage amplitude of a signal is not provided in the presentembodiment, such a level shift may be appropriately provided by adesigner.

The digital video signal (DV) to be supplied externally to the sourcesignal line driver circuit is input into the latch (A) 102-2 through theillustrated wirings. A latch signal S_LAT and a signal S_LATb obtainedby inverting the polarity of the S_LAT are respectively input into thelatch (B) 102-3 through the illustrated wirings.

The detailed structure of the latch (A) 102-2 is the same as that shownin FIG. 14. Accordingly, the description thereof is omitted here.

Reference numeral 108 denotes the start pulse signal control circuit,that can supply a constant electrical potential (fixed electricalpotential), instead of the start pulse signal is (SP), to the shiftregister 102-1 for a constant time period.

More specifically, the constant electrical potential (fixed electricalpotential) instead of the start pulse signal is input into the shiftregister 102-2 for a constant time period by means of the start pulsesignal control circuit 108, so that the timing signal that is to be usedfor inputting the digital video signal at less significant bits in therange from the first bit to the m-th bit into the latch (A) 102-2 isprevented from being input into the shift register 102-1. Accordingly,only the digital video signal at more significant bits in the range fromthe (m+1)-th bit to the n-th bit can be written into the latch (A)102-2.

A structure of the start pulse signal control circuit 108 in the presentembodiment is the same as those shown in FIGS. 18A and 18B. Accordingly,the detailed description about the structure of the start pulse signalcontrol circuit 108 should be found in Embodiment 4. It should be noted,however, that in the present embodiment, the start pulse signal is inputinto the input terminal (IN) in the circuits as shown in FIGS. 18A and18B. A signal output from an output terminal (OUT) in the circuits asshown in FIGS. 18A and 18B is input into the shift register 102-1. Theselection signal determines whether the start pulse signal is to beinput into the shift register 102-1, or the constant electricalpotential (fixed electrical potential) is to be instead suppliedthereto.

The structure of the start pulse signal control circuit is not limitedto those shown in FIGS. 18A and 18B.

The present embodiment can be freely combined with Embodiments 1 through3.

Embodiment 7

In the present embodiment, another example for the third structure inaccordance with the present invention, which is different from that asdescribed in Embodiment 5, will be described with reference to FIG. 21.In FIG. 21, the same component as shown in FIG. 5 are designated withthe same reference numerals.

In FIG. 21, reference numeral 501 denotes a power source line, referencenumeral 502 denotes a buffer amplifier, reference numeral 503 denotes amonitoring light emitting device, reference numeral 504 denotes aconstant current source, and reference numeral 505 denotes an addingcircuit. One of electrodes of the monitoring light emitting element 503is connected to the constant current source 504, so that a constantcurrent always flows through the monitoring light emitting element 503.When a temperature of an organic compound layer contained in the lightemitting element changes, the magnitude of the current to flow throughthe monitoring light emitting element 503 does not change, but rather,an electrical potential of the electrode of the monitoring lightemitting element 503 connected to the constant current source 504changes.

On the other hand, the buffer amplifier 502 includes two input terminalsand one output terminal. One of the two input terminals is anon-inverted input terminal (+), while the other is an inverted inputterminal (−). An electrical potential at one of electrodes of themonitoring light emitting device 503 is supplied to the non-invertedinput terminal of the buffer amplifier 502.

The buffer amplifier 502 is a circuit for preventing an electricalpotential at a pixel electrode of the monitoring light emitting element503 connected to the constant current source 504 from changing inaccordance with a load such as a wiring capacitance of the power sourceline 501 or the like. Accordingly, the electrical potential provided tothe non-inverted input terminal of the buffer amplifier 502 is outputfrom an output terminal to be supplied to the power source line as thepower source potential, without being changed in accordance with a loadof a wiring capacitance of the power source line 501, adding circuit 505or the like, to the adding circuit 505.

The electrical potential supplied to the adding circuit 505 from theoutput terminal of the buffer amplifier 502 is further supplied to thepower source line 501 as the power source voltage, after a certainconstant potential difference is added thereto or subtracted therefrom.

FIG. 22 shows the detailed circuit diagram of the adding circuit in thepresent embodiment. The adding circuit 505 includes a first resistor521, a second resistor 522, a power source 525 for the adding circuit,and a non-inverting amplifier circuit 520. The non-inverting amplifiercircuit 520 includes a third resistor 523, a fourth resistor 524, apower source 526 for the non-inverting amplifier circuit, and anamplifier 527.

One of the terminals of the first resistor 521 functions as an inputterminal (IN) of the adding circuit. The other terminal of the firstresistor 521 is connected to one of the terminals of the second resistor522. The other terminal of the second resistor 522 is connected to thepower source 525 for the adding circuit. An output obtainable betweenthe first resistor 521 and the second resistor 522 is input into thenon-inverted input terminal (+) of the amplifier 527 of thenon-inverting amplifier circuit 520.

One of the terminals of the third resistor 523 is connected to an outputterminal of the amplifier 527, while the other terminal of the thirdresistor 523 is connected to the inverted input terminal of theamplifier 527. An output obtainable between the third resistor 523 andthe inverted input terminal of the amplifier 527 is input to one of theterminals of the fourth resistor 524. The other terminal of the fourthresistor 524 is connected to the power source 526 for the non-invertingamplifier circuit. An output obtainable between the third resistor 523and the output terminal of the amplifier 527 is output from an outputterminal (OUT) of the adding circuit 505.

In accordance with the above-described structure, even when atemperature of the monitoring light emitting element 503 or the organiccompound layer of the light emitting element in the pixel portionchanges due to a change in an environmental temperature, the powersource potential is changed so as to allow a constant current to flowthrough the light emitting element. Thus, even when the environmentaltemperature of the light emitting device increases, power consumption ofthe light emitting device can be prevented from increasing, andfurthermore, the brightness of the light emitting element can bemaintained at a constant level. Moreover, by further providing theadding circuit 505, the electrical potential of the power source line501 is not required to be at the same level as the electrical potentialof the electrode connected to the constant current source 504 of themonitoring light emitting element 503. Thus, the magnitude of thecurrent to flow through the buffer amplifier 502, the monitoring lightemitting element 503, and the constant current source 504 can besuppressed, thereby resulting in the power consumption being suppressed.

The structure of the adding circuit 505 is not limited to that shown inFIG. 22.

The present embodiment can be freely combined with Embodiments 1 through6.

Embodiment 8

In Embodiment 8, a method of manufacturing a pixel portion and TFTs(n-channel TFTs and p-channel TFTs) of a driver circuit formed in theperiphery of the pixel portion, on the same substrate simultaneously isexplained in detail.

First, as shown in FIG. 23A, a base film 401 made from an insulatingfilm such as a silicon oxide film, a silicon nitride film, or a siliconoxynitride film is formed on a substrate 400 made from glass such asbarium borosilicate glass or aluminum borosilicate glass, typicallyCorning Corp. #7059 glass or #1737 glass, or made from a quartzsubstrate. For example, a silicon oxynitride film made from SiH₄, NH₃,and N₂O by plasma CVD is formed with a thickness of 10 to 200 nm(preferably from 50 to 100 nm), and a hydrogenated silicon oxynitridefilm with a thickness of 50 to 200 nm (preferably between 100 and 150nm), made from SiH₄ and N₂O, is similarly formed and laminated. Notethat the base film is shown as one layer in FIG. 23A. The base film 401is shown as a two layer structure in Embodiment 8, but it may also beformed as a single layer of the above insulating films, and it may alsobe formed having a lamination structure in which two layers or more arelaminated.

Semiconductor layers 402 to 405 are formed from a crystallinesemiconductor film which is formed by a semiconductor film having anamorphous structure with a laser crystallization method or a knownthermal crystallization method. The thickness of the semiconductorlayers 402 to 405 is formed to a thickness of 25 to 80 nm (preferablybetween 30 and 60 nm). There are no limitations in the crystallinesemiconductor film material, but it is preferable to form the film fromsilicon or silicon germanium (SiGe) alloy.

As for known crystallization methods, there is a thermal crystallizationmethod using an electric furnace, a laser annealing crystallizationmethod using laser light, a lamp annealing crystallization method usinginfrared light, and a crystallization method using a catalyst metal.

A laser such as a pulse emission type or continuous emission typeexcimer laser, a YAG laser, and a YVO₄ laser can be used in the lasercrystallization method to manufacture a crystalline semiconductor film.A method of condensing laser light emitted from a laser emission deviceinto a linear shape by an optical system and then irradiating the lightto the semiconductor film may be used when these types of lasers areused. The crystallization conditions may be suitably selected by theoperator, but when using the excimer laser, the pulse emission frequencyis set to 300 Hz, and the laser energy density is set from 100 to 400mJ/cm² (typically between 200 and 300 mJ/cm²). Further, the secondharmonic is utilized when using the YAG laser, the pulse emissionfrequency is set from 30 to 300 kHz, and the lasers energy density maybe set from 300 to 600 mJ/cm² (typically between 350 and 500 mJ/cm²).The laser light collected into a linear shape with a width of 100 to1000 μm, for example 400 μm, is then irradiated over the entire surfaceof the substrate. This is performed with an overlap ratio of 50 to 98%for the linear shape laser light.

Then, a gate insulating film 406 is formed covering the semiconductorlayers 402 to 405. A gate insulating film 406 is formed by an insulatingfilm containing silicon with a thickness of 40 to 150 nm by plasma CVDor sputtering. A 120 nm thick silicon oxynitride film is formed inEmbodiment 8. The gate insulating film 406 is not limited to this typeof silicon oxynitride film, of course, and other insulating filmscontaining silicon may also be used, in a single layer or in alamination structure. For example, when using a silicon oxide film, itcan be formed by plasma CVD with a mixture of TEOS (tetraethylorthosilicate) and O₂, at a reaction pressure of 40 Pa, with thesubstrate temperature set from 300 to 400° C., and by discharging at ahigh frequency (13.56 MHZ) electric power density of 0.5 to 0.8 W/cm².Good characteristics as a gate insulating film can be obtained bysubsequently performing thermal annealing, at between 400 and 500° C.,of the silicon oxide film thus manufactured.

A first conducting film 407 and a second conducting film 408 are thenformed on the gate insulating film 406 in order to form gate electrodes.The first conducting film 407 is formed from Ta (tantalum) with athickness of 50 to 100 nm, and the second conducting film 408 is formedfrom W (tungsten) having a thickness of 100 to 300 nm, in Embodiment 8.

The Ta film is formed by sputtering, and sputtering of a Ta target isperformed in Ar. If appropriate amounts of Xe and Kr are added to Ar atthe time of sputtering, the internal stress of the Ta film is relaxed,and film peeling can be prevented. The resistivity of an α phase Ta filmis on the order of 20 Ωcm, and it can be used in the gate electrode, butthe resistivity of a β phase Ta film is on the order of 180 μΩcm and itis unsuitable for the gate electrode. An α phase Ta film can easily beobtained if a tantalum nitride film, which possesses a crystal structurenear that of α phase Ta, is formed with a thickness of 10 to 50 nm as abase for Ta in order to form α phase Ta.

The W film is formed by sputtering with a W target, which can also beformed by thermal CVD using tungsten hexafluoride (WF₆). Whichever isused, it is necessary to be able to make the film become low resistancein order to use it as the gate electrode, and it is preferable that theresistivity of the W film be made equal to or less than 20 μΩcm. Theresistivity can be lowered by enlarging the crystal grains of the Wfilm, but for cases in which there are many impurity elements such asoxygen in the W film, crystallization is inhibited, and the film becomeshigh resistance. A W target having a purity of 99.9999% or 99.99% isthus used in sputtering. In addition, by forming the W film while takingsufficient care that no impurities from the gas phase are introduced atthe time of film formation, the resistivity of 9 to 20 μΩcm can beachieved.

Note that, although the first conducting film 407 is Ta and the secondconducting film 408 is W in Embodiment 8, the conducting films are notlimited to these, and both may also be formed from an element selectedfrom the group consisting of Ta, W, Ti, Mo, Al, and Cu, or from an alloymaterial having one of these elements as its main constituent, or from achemical compound of these elements. Further, a semiconductor film,typically a poly-crystalline silicon film into which an impurity elementsuch as phosphorus is doped, may also be used. An example of preferablecombinations other than that used in Embodiment 8 include: forming thefirst conducting film by tantalum nitride (TaN) and combining it withthe second conducting film formed from W; forming the first conductingfilm by tantalum nitride (TaN) and combining it with the secondconducting film formed from Al; and forming the first conducting film bytantalum nitride (TaN) and combining it with the second conducting filmformed from Cu. (See FIG. 23B.)

Masks 409 to 412 are formed next from resist, and a first etchingprocess is performed in order to form electrodes and wirings. An ICP(inductively coupled plasma) etching method is used in Embodiment 8. Agas mixture of CF₄ and Cl₂ is used as an etching gas, and a plasma isgenerated by applying a 500 W RF electric power (13.56 MHZ) to a coilshape electrode at a pressure of 1 Pa. A 100 W RF electric power (13.56MHZ) is also applied to the substrate side (test piece stage),effectively applying a negative self-bias voltage. The W film and the Tafilm are both etched on the same order when CF₄ and Cl₂ are combined.

Not shown in FIG. 23C, edge portions of the first conducting layer andthe second conducting layer are made into a tapered shape in accordancewith the effect of the bias voltage applied to the substrate side underthe above etching conditions by using a suitable resist mask shape. Theangle of the tapered portions is from 15 to 45°. The etching time may beincreased by approximately 10 to 20% in order to perform etching withoutany residue remaining on the gate insulating film. The selectivity of asilicon oxynitride film with respect to a W film is from 2 to 4(typically 3), and therefore approximately 20 to 50 nm of the exposedsurface of the silicon oxynitride film is etched by this over-etchingprocess. Further, not shown in FIG. 23C, regions of the gate insulatingfilm 406 not covered by first shape conducting layers 414 to 417 aremade thinner by 20 to 50 nm after etching.

The first shape conducting layers 414 to 417 (first conducting layers414 a to 417 a and second conducting layers 414 b to 417 b) are thusformed from the first conducting layer and the second conducting layerin accordance with the first etching process.

A second etching process is performed next, as shown in FIG. 23D. TheICP etching method is similarly used, a mixture of CF₄, Cl₂, and O₂ isused as the etching gas, and a plasma is generated by supplying a 500 WRF electric power (13.56 MHZ) to a coil shape electrode at a pressure of1 Pa. A 50 W RF (13.56 MHZ) electric power is applied to the substrateside (test stage), and a self-bias voltage which is lower in comparisonto that of the first etching process is applied. The W film is etchedanisotropically under these etching conditions, and Ta (the firstconducting layers) is anisotropically etched at a slower etching speed,forming second shape conducting layers 419 to 422 (first conductinglayers 419 a to 422 a and second conducting layers 419 b to 422 b).Further, although not shown in FIG. 23D, the gate insulating film 406 isadditionally etched on the order of 20 to 50 nm, becoming thinner, inregions not covered by the second shape conducting layers 419 to 422.

The etching reaction of the W film and the Ta film in accordance withthe mixed gas of CF₄ and Cl₂ can be estimated from the radicalsgenerated, and from the ion types and vapor pressures of the reactionproducts. Comparing the vapor pressures of W and Ta fluorides andchlorides, the W fluoride compound WF₆ is extremely high, and the vaporpressures of WCl₅, TaF₅, and TaCl₅ are of similar order. Therefore the Wfilm and the Ta film are both etched by the CF₄ and Cl₂ gas mixture.However, if a suitable quantity of O₂ is added to this gas mixture, CF₄and O₂ react, forming CO and F, and a large amount of F radicals or Fions are generated. As a result, the etching speed of the W film havinga high fluoride vapor pressure becomes fast. On the other hand, even ifF increases, the etching speed of Ta does not relatively increase.Further, Ta is easily oxidized compared to W, and therefore the surfaceof Ta is oxidized by the addition of O₂. The etching speed of the Tafilm is further reduced because Ta oxides do not react with fluorine andchlorine. It therefore becomes possible to have a difference in etchingspeeds between the W film and the Ta film, and it becomes possible tomake the etching speed of the W film larger than that of the Ta film.

Then, the masks 409 a to 412 a are removed, and a first doping processis performed as shown in FIG. 24A, adding an impurity element whichimparts n-type conductivity. For example, doping may be performed at anacceleration voltage of 70 to 120 keV and with a dose amount of 1×10¹³atoms/cm². The doping process is performed using the second shapeconducting layers 419 to 422 as masks against the impurity element, andso as to also add the impurity element in regions below the secondconducting layers 419 a to 422 a. First impurity regions 425 to 428,which overlap with the second conducting layers 419 a to 422 a, andsecond impurity regions 429 to 432, which have a higher impurityconcentration than the first impurity regions, are thus formed. Notethat the impurity element which imparts n-type conductivity is addedafter removing the masks 409 a to 412 a in Embodiment 8, but the presentinvention is not limited to this. The impurity element which impartsn-type conductivity may also be added in the step of FIG. 24A, and thenthe masks 409 a to 412 a may be removed.

A mask 433 is next formed on the semiconductor layer 404 so as to coverthe second conducting layers 421 a and 421 b. The mask 433 partiallyoverlaps with the second impurity region 431, sandwiching the gateinsulating film 406. A second doping process is then performed, and animpurity element which imparts n-type conductivity is added. Doping ofthe impurity element which imparts n-type conductivity is performed atconditions in which the dose amount is raised higher than that of thefirst doping process, and at a low acceleration voltage. (See FIG. 24B.)The doping can be carried out by ion doping or ion implantation. Iondoping is performed under conditions of a dose amount from 1×10¹³ to5×10¹⁴ atoms/cm² and an acceleration voltage of 60 to 100 keV. Aperiodic table group 15 element, typically phosphorus (P) or arsenic(As) is used as the impurity element which imparts n-type conductivity,and phosphorus (P) is used here. The second shape conducting layers 419to 422 become masks with respect to the impurity element which impartsn-type conductivity in this case, and source regions 434 to 437, drainregions 438 to 441, and Lov regions 442 to 445 are formed in aself-aligning manner. Further, Loff region 446 is formed in accordancewith the mask 433. The impurity element which imparts n-typeconductivity is added to the source regions 434 to 437, and to the drainregions 438 to 441 with a concentration in the range of 1×10²⁰ to 1×10²¹atoms/cm³.

It is possible to freely set the length of the Loff region 446 bycontrolling the size of the mask 433 according to Embodiment 8.

Note that in the specification, the LDD region overlapping with a gateelectrode through a gate insulating film is referred to as an Lovregion, and the LDD region not overlapping with a gate electrode througha gate insulating film is referred to as an Loff region.

The impurity element which imparts n-type conductivity is added at aconcentration of 1×10¹⁷ to 1×10¹⁹ atoms/cm³ in the Loff region, and at aconcentration of 1×10¹⁶ to 1×10¹⁸ atoms/cm³ in the Lov region.

Note that, in FIG. 24B, either before or after doping of an impurityelement which imparts n-type conductivity is performed under the abovementioned condition, doping of an impurity element which imparts ann-type conductivity may also be performed with an acceleration voltageof 70 to 20 keV in a state in which the mask 433 is formed on thesemiconductor layer 404. The concentration of the impurity element whichimparts an n-type conductivity in a portion 446 which becomes an Loffregion of the switching TFT can be suppressed in accordance with theabove process, and the concentration of the impurity element whichimparts n-type conductivity in portions 442 and 443, which become Lovregions of the TFTs used in the driver circuit can be increased. It ispossible to lower the off current of the switching TFT by suppressingthe concentration of the impurity element which imparts an n-typeconductivity in the portion 446 which becomes the Loff region of theswitching TFT. Further, hot carriers generated in accordance with a highelectric field in the vicinity of the drain and a cause of a degradationphenomenon due to the hot carrier effect can be prevented by increasingthe concentration of the n-type conductivity imparting impurity elementin the portion 443 which becomes the Lov region of the n-channel TFTused in the driver circuit.

After removing the mask 453, source regions 447 and 448, drain regions449 and 450, and Lov regions 451 and 452, having a conductivity typewhich is the inverse of the above one conductivity type, are then formedin the semiconductor layers 402 and 405 for forming the p-channel TFT,as shown in FIG. 24C. The second shape conducting layers 419 and 422 areused as a mask with respect to the impurity element, and the impurityregions are formed in a self-aligning manner. The semiconductor layers402 and 403, which form n-channel TFTs, are covered over their entiresurface areas by a resist masks 453 at this point. Phosphorus is addedin differing concentration to the source regions 447 and 448, the drainregions 449 and 450, and the Lov regions 451 and 452, and ion doping isperformed here using diborane (B₂H₆), so that impurity is added to eachof the regions with a concentration of 2×10²⁰ to 2×10²¹ atoms/cm³.

Impurity regions (source regions, drain regions, Lov regions, and Loffregions) are formed in the respective semiconductor layers 402 to 405 bythe above processes. The second conducting layers 419 to 422 overlappingthe semiconductor layers function as gate electrodes.

A process of activating the impurity elements added to the respectivesemiconductor layers is then performed, with the aim of controlling theconductivity type. Thermal annealing using an annealing furnace isperformed for this process. In addition, laser annealing and rapidthermal annealing (RTA) can also be applied. Thermal annealing isperformed with an oxygen concentration equal to or less than 1 ppm,preferably equal to or less than 0.1 ppm, in a nitrogen atmosphere at400 to 700° C., typically between 500 and 600° C. Heat treatment isperformed for 4 hours at 500° C. in Embodiment 8. However, for cases inwhich the wiring material used in the conducting layers 419 to 422 isweak with respect to heat, it is preferable to perform activation afterforming an interlayer insulating film (having silicon as its mainconstituent) in order to protect the wirings and the like.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an atmosphere containing between 3 and 100% hydrogen,performing hydrogenation of the semiconductor layers. This process isone of terminating dangling bonds in the semiconductor layers byhydrogen which is thermally excited. Plasma hydrogenation (usinghydrogen excited by a plasma) may also be performed as another means ofhydrogenation.

A first interlayer insulating film 455 is formed next from a siliconoxynitride film having a thickness of 100 to 200 nm. (FIG. 25A) A secondinterlayer insulating film 458 made from an organic insulating materialis then formed on the first interlayer insulating film 455.

Contact holes are then formed in the gate insulating film 406, the firstinterlayer insulating film 455 and the second interlayer insulating film458, and source wirings 459 to 462 are formed to contact the sourceregions 447, 435, 436, and 448 through the contact holes. In the sameway, drain wirings 463 to 465 are further formed to contact the drainregions 449, 439, 440 and 450. (FIG. 25B)

Note that it is preferable to form the contact holes by dry etchingusing CF₄ and O₂ when the gate insulating film 406, the first interlayerinsulating film 455, and the second interlayer insulating film 458 areSiO₂ films or SiON films. Further, for cases in which the gateinsulating film 406, the first interlayer insulating film 455, and thesecond interlayer insulating film 458 are organic resin films, it ispreferable to form the contact holes by dry etching using CHF₃ or by BHF(buffered hydrogen fluoride, HF+NH₄F). In addition, if the gateinsulating film 406, the first interlayer insulating film 455 and thesecond interlayer insulating film 458 are formed by different materials,it is preferable to change the method of etching and the etchant oretching gas type for each film. The contact holes may also be formed byusing the same etching method and the same etchant or etching gas.

A third interlayer insulating film 467 is formed next from an organicresin. Organic resins such as polyimide, polyamide, acrylic, and BCB(benzocyclobutene) can be used. In particular, it is preferable to useacrylic, which has superior levelness, because the third interlayerinsulating film 467 is formed with a strong implication of leveling. Anacrylic film is formed in Embodiment 8 at a film thickness at whichsteps formed by the TFTs can be sufficiently leveled. The film thicknessis preferably from 1 to 5 μm (more preferably between 2 and 4 μm).

A contact hole for reaching the drain wiring 465 is formed next in thethird interlayer insulating film 467, and a pixel electrode 468 isformed. An indium tin oxide (ITO) film is formed with a thickness of 110nm in Embodiment 8, and patterning is then performed, thereby formingthe pixel electrode 468. Further, a transparent conducting film in whichbetween 2 and 20% zinc oxide (ZnO) is mixed with indium oxide may alsobe used. The pixel electrode 468 becomes an anode of a light emittinglayer. (See FIG. 25C.)

A first bank 469 and a second bank 470 are formed next from a resinmaterial. The first bank 469 and the second bank 470 are formed in orderto separate an organic compound layers and cathodes, which are formedlater, of adjacent pixels. It is therefore preferable that the secondbank 470 stick out farther horizontally than the first bank 469. Notethat it is preferable that the combined thickness of the first bank 469and the second bank 470 be made on the order of 1 to 2 μm, but there areno limitations on this thickness provided that the organic compoundlayers and the cathodes formed later of adjacent pixels can beseparated. Further, it is necessary to form the first bank 469 and thesecond bank 470 by an insulating film, and it is therefore possible touse materials such as an oxide or a resin, for example. The first bank469 and the second bank 470 may both be formed by the same material, andthey may also be formed by different materials. The first bank 469 andthe second bank 470 are formed in stripe shapes between pixels. Thefirst bank 469 and the second bank 470 may be formed on and along thesource wirings (source signal lines), and may be formed on and along thegate wirings (gate signal lines). Note that the first bank 469 and thesecond bank 470 may also be formed by a material in which a pigment ismixed into a resin. (See FIG. 26A.)

An organic compound layer 471 and a cathode (MgAg electrode) 472 areformed next in succession without exposure to the atmosphere usingvacuum evaporation. Note that the film thickness of the organic compoundlayer 471 may be from 80 to 200 nm (typically between 100 and 120 nm),and that the film thickness of the cathode 472 may be from 180 to 300 nm(typically between 200 and 250 nm). Note also that, although only onepixel is shown in Embodiment 8, an organic compound layer which emitsred color light, an organic compound layer which emits green colorlight, and an organic compound layer which emits blue color light areformed at the same time at this point. Note that materials to form anorganic compound layer and a cathode is partially laminated on the bank470, however, in this specification, the materials are not included inthe organic compound layer 471 and the cathode 472.

The organic compound layer 471 and the cathode 472 are formed in orderfor a pixel corresponding to the red color, a pixel corresponding to thegreen color, and a pixel corresponding to the blue color. However, theorganic compound layer 471 lacks resistance with respect to solutions,and therefore each color must be formed separately without using aphotolithography technique. It is preferable to use a metal mask andcover the pixels other than the desired pixel, and selectively form theorganic compound layer 471 and the cathode 472 in only the requiredportions.

Namely, first a mask is set so as to cover all of the pixels except forthose corresponding to the red color, and red color light-emittingorganic compound layers are selectively formed using the mask. Next, amask is set so as to cover all of the pixels except for thosecorresponding to the green color, and green color light-emitting organiccompound layers are selectively formed using the mask. Finally, a maskis set so as to cover all of the pixels except for those correspondingto the blue color, and blue color light-emitting organic compound layersare selectively formed using the mask. Note that, although the use ofall different masks is described here, the same mask may also be reused.Further, it is preferable to perform processing until an organiccompound layer and a cathode are formed on all of pixels withoutreleasing the vacuum.

Note that the organic compound layer 471 has a single layer structurecomposed of only a light-emitting layer is shown in Embodiment 8, but astructure having layers such as a hole transporting layer, a holeinjecting layer, an electron transporting layer, and an electroninjecting layer in addition to the light-emitting layer may also be usedfor the organic compound layer. Various examples of these types ofcombinations have already been reported, and all such structures may beused. A known material can be used as the organic compound layer 471.Considering the driver voltage of a light emitting element, it ispreferable to use an organic material as the known material.

The cathode 472 is formed next. An example of using an MgAg electrode asthe cathode of an light emitting element is shown in Embodiment 8, butit is also possible to use other known materials.

The active matrix substrate having the structure shown in FIG. 26B isthus completed. Note that, after forming the first bank 469 and thesecond bank 470, it is effective to perform processing in successionwithout exposure to the atmosphere up through to the formation of thecathode 472 by using a multi-chamber method (or an in-line method) thinfilm formation apparatus.

In Embodiment 8, a source region 504, a drain region 505, an Loff region506, an Lov region 507, and a channel forming region 508 are containedin a semiconductor layer of a switching TFT 501. The Loff region 506 isformed so as not to overlap with the gate electrode 421 through the gateinsulating film 406. Further, the Lov region 507 is formed so as tooverlap with the gate electrode 421 through the gate insulating film406. This type of structure is extremely effective in reducing the offcurrent.

Further, a single gate structure is used as the switching TFT 501 inEmbodiment 8, but the present invention may also have a double gatestructure or another type of multi-gate structure for the switching TFT.Two TFTs are substantially connected in series by using the double gatestructure, giving the advantage of additionally reducing the offcurrent.

Further, the switching TFT 501 is an n-channel TFT in Embodiment 8, buta p-channel TFT may also be used.

A semiconductor layer of a current controlling TFT 502 contains a sourceregion 510, a drain region 511, an Lov region 512, and a channel formingregion 513. The Lov region 512 is formed so as to overlap with the gateelectrode 422 through the gate insulating film 406. Note that thecurrent controlling TFT 502 does not have the Loff region in Embodiment8, but a structure having the Loff region may also be used.

Further, the current controlling TFT 502 is a p-channel TFT inEmbodiment 8, but it may also be an n-channel TFT.

Note that the active matrix substrate of Embodiment 8 shows an extremelyhigh reliability, and its operational characteristics are alsoincreased, by arranging optimally structured TFT in not only the pixelportion, but also in the driver circuit portion.

First, a TFT having a structure in which hot carrier injection isreduced so as not to have a very large drop in operational speed is usedas an n-channel TFT 503 of a CMOS circuit forming the driver circuitportion. Note that circuits such as a shift register, a buffer, a levelshifter, and a sampling circuit (sample and hold circuits) are includedas the driver circuits here. Signal conversion circuits such as a D/Aconverter can also be included in the case of performing digital drive.

A semiconductor layer of the n-channel TFT 503 of the CMOS circuit inEmbodiment 8 contains a source region 521, a drain region 522, an Lovregion 523, and a channel forming region 524.

Further, a semiconductor layer of a p-channel TFT 504 of the CMOScircuit contains a source region 531, a drain region 532, an Lov region533, and a channel forming region 534.

Note that, in practice, it is preferable to perform packaging (sealing)by a protecting film having high airtight characteristics and littleoutgassing (such as a laminate film or an ultraviolet hardened resinfilm) or by a transparent sealing material after completing up throughto the processes of FIG. 26B so as to have no exposure to theatmosphere. Further, if an inert gas is placed in the inside of thesealing material, and a drying agent (barium oxide, for example) isarranged inside of the sealing material, then the reliability of thelight emitting element is increased.

Further, a connector (flexible printed circuit, FPC) is attached inorder to connect the elements formed on the substrate, with terminalsextended from the circuits, to external signal terminals afterincreasing the airtight characteristics in accordance with the packagingprocess or the like. A manufactured product is thus completed. This typeof deliverable state is referred to as a light emitting devicethroughout this specification.

The widths of the gate electrodes in the direction of the channel length(referred to hereinafter as a width of the gate electrode) differ asstated above in accordance with manufacturing processes of the presentinvention. Therefore, it is possible to make the ion implantation withinthe semiconductor layers arranged under the first gate electrode lessthan the ion concentration within the semiconductor layers not arrangedunder the first gate electrode by utilizing the difference in ionpenetration depth, due to the difference of gate electrode thickness,when performing ion injection using the gate electrodes as masks.

Further, in order to form the Loff regions using a mask, only the widthof Lov region needs to be controlled by etching. It becomes easy tocontrol positions of the Lov regions and the Loff regions.

Note that although an example in which light emitted from the organiccompound layer is directed toward the substrate side is explained inEmbodiment 8, the present invention is not limited to this, and astructure in which light emitted from the organic compound layer isdirected above the substrate may also be used. In this case, the cathodeof the light emitting element becomes the pixel electrode, and it ispreferable that the current controlling TFT be an n-channel TFT.

Note that although the case in which a pixel has a switching TFT and acurrent controlling TFT is explained in Embodiment 8, the presentinvention is not limited to this. Even when a pixel has three TFTs ormore, it is possible to apply the present embodiment.

The method of manufacturing a light emitting device according to thepresent invention is not limited to the manufacturing method describedin Embodiment 8 and other manufacturing methods can be utilized.

Note that it is possible to freely combine Embodiment 8 with any ofEmbodiments 1 to 7.

Embodiment 9

A light emitting device manufactured by the present invention hassuperior visibility in bright locations in comparison to a liquidcrystal display device because it is a self-emission type device, andmoreover viewing angle is wide. Accordingly, it can be used as a displayportion for various electronic apparatuses. For example, it isappropriate to use the light emitting display device of the presentinvention as a display portion of a display device incorporating thelight emitting device in its casing having a diagonal equal to 30 inchesor greater (typically equal to 40 inches or greater) for appreciation ofTV broadcasts by large screen. The light emitting device of the presentinvention can be used as a display portion for various electronicapparatuses.

The following can be given as examples of such electronic apparatuses: avideo camera; a digital camera; a goggle type display (head mounteddisplay); a car navigation system; an audio reproducing device (such asa car audio system, an audio compo system); a notebook personalcomputer; a game equipment; a portable information terminal (such as amobile computer, a mobile telephone, a mobile game equipment or anelectronic book); and an image playback device provided with a recordingmedium (specifically, a device which performs playback of a recordingmedium and is provided with a display which can display those images,such as a digital video disk (DVD)). In particular, because portableinformation terminals are often viewed from a diagonal direction, thewideness of the field of vision is regarded as very important. Thus, itis preferable that the light emitting device is employed. Examples ofthose electronic apparatuses are shown in FIGS. 27 and 28.

FIG. 27A illustrates a portable information terminal which includes adisplay panel 2701 and an operation panel 2702. The display panel 2701is connected with the operation panel 2702 at a connection portion 2703.In the connection portion 2703, an angle θ between a surface providedwith the display portion 2704 of the display panel 2701 and a surfaceprovided with an operation key 2706 of the operation panel 2702 can bearbitrary varied.

The display panel 2701 includes the display portion 2704. Further, theportable information terminal shown in FIG. 27A has a function as atelephone, and the display panel 2701 includes an audio output portion2705, so that voice is outputted from the audio output portion 2705. Thelight emitting device of the present invention can be utilized for thedisplay portion 2704.

The operation panel 2702 includes an operation key 2706, a power switch2707, an audio input portion 2708, and a CCD receiving portion 2709.Note that although the operation key 2706 and the power switch 2707 areprovided separately in FIG. 27A, the power switch 2707 may be includedin the operation key 2706.

In the audio input portion 2707, voice is inputted. The image inputtedat the CCD receiving portion 2709 is received in the portableinformation terminal as an electronic data.

Note that although the display panel 2701 includes the audio outputportion 2705 and the operation panel includes the audio input portion2708 in FIG. 27A, the present embodiment is not limited to this. Namely,the display panel 2701 includes the audio input portion 2708, and theoperation panel includes the audio output portion 2705. Further, theaudio output portion 2705 and the audio input portion 2708 may beprovided in the display panel 2701, and the audio output portion 2705and the audio input portion 2708 may be provided in the operation panel2702.

Note that although the portable information terminal includes no antennain FIG. 27A, an antenna may be provided, if necessary.

FIG. 27B illustrates a portable telephone, which includes a main body2601, an audio output portion 2602, an audio input portion 2603, adisplay portion 2604, operation switches 2605, and an antenna 2606. Thelight emitting device in accordance with the present invention can beused as the display portion 2604. The display portion 2604 can reducepower consumption of the portable telephone by displaying white-coloredcharacters on black-colored background.

The light emitting device of the present invention is very effective forthe portable type electronic apparatus since power consumption can bereduced.

FIG. 28A illustrates a display device having a light emitting devicewhich includes a frame 2001, a support table 2002, a display portion2003, or the like. The light emitting device of the present invention isapplicable to the display portion 2003. The light emitting device is ofthe self-emission type and therefore requires no back light. Thus, thedisplay portion thereof can have a thickness thinner than that of theliquid crystal display device.

FIG. 28B illustrates a video camera which includes a main body 2101, adisplay portion 2102, an audio input portion 2103, operation switches2104, a battery 2105, an image receiving portion 2106, or the like. Thelight emitting device in accordance with the present invention can beused as the display portion 2102.

FIG. 28C illustrates one portion (the right-half piece) of ahead-mounted type electronic apparatus which includes a main body 2201,signal cables 2202, a head mount band 2203, a screen portion 2204, anoptical system 2205, a display portion 2206, or the like. The lightemitting device of the present invention is applicable to the displayportion 2206.

FIG. 28D illustrates an image reproduction apparatus provided with arecording medium (more specifically, a DVD reproduction apparatus),which includes a main body 2301, a recording medium (a DVD or the like)2302, operation switches 2303, a display portion (a) 2304, anotherdisplay portion (b) 2305, or the like. The display portion 2304 (a) isused mainly for displaying image information, while the display portion2305 (b) is used mainly for displaying character information. The lightemitting device in accordance with the present invention can be used asthese display portions 2304(a) and 2305(b). The image reproductionapparatus provided with a recording medium further includes a domesticgame equipment or the like.

FIG. 28E illustrates a goggle type display (head-mounted display) whichincludes a main body 2401, a display portion 2402, and an arm portion2403. The light emitting device in accordance with the present inventioncan be used to the display portion 2402.

FIG. 28F illustrates a personal computer which includes a main body2501; a frame 2502, a display portion 2503, a key board 2504, or thelike. The light emitting device in accordance with the present inventioncan be used as the display portion 2503. Note that if emission luminanceof an organic material becomes higher in the future, light includingoutputted image information is enlarged to be projected by means oflenses or the like, thereby applying to a front-type or a rear-typeprojector.

The aforementioned electronic apparatuses are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system), and in particular likely todisplay moving picture information. The light emitting device issuitable for displaying moving pictures since the organic material canexhibit high response speed.

Since a light emitting portion of the light emitting device consumespower, it is desirable to display information in such a manner that thelight emitting portion therein becomes as small as possible.Accordingly, when the light emitting device is applied to a displayportion which mainly displays character information, e.g., a displayportion of a portable information terminal, and more particular, aportable telephone or a car audio reproducing equipment, it is desirableto drive the light emitting display device so that the characterinformation is formed by a light-emitting portion while a non-emissionportion corresponds to the background.

As set forth above, the present invention can be applied variously to awide range of electronic apparatuses in all fields. Note that it ispossible to freely combine Embodiment 9 with any of Embodiments 1 to 8.

Embodiment 10

In this embodiment, a further specific structure of a third structure ofthe present invention and a change in brightness due to a temperaturewill be explained with measured values.

FIG. 29A shows a connection state of a light emitting element for amonitor, which is included in a light emitting device of thisembodiment. Reference numerals 701, 702, 703, 704 and 705 represents 4power source line, a buffer amplifier, a monitoring light emittingelement, a constant current source, and one of light emitting elementsin a pixel portion, respectively.

FIG. 29A shows a case when a current controlling TFT (not shown) is “ON”state, and the current controlling TFT directly connects a pixelelectrode of the light emitting element 705 in the pixel portion and thepower source line 701.

Also, in FIG. 29A, although an anode of the light emitting element 705in the pixel portion is used as a pixel electrode, this embodiment isnot limited to such a structure. A cathode may be used as a pixelelectrode.

The constant current source 704 of this embodiment has an amplifier, avariable resistor and a bipolar transistor. References V1 and V2represent a predetermined voltage to be applied, a relationship (avoltage applied to an anode<V2<V1) is satisfied. By the way, therelationship among the voltage applied to the anode, V2, and V1 ischanged depending on whether an anode is used as a pixel electrode or acathode is used as pixel electrode. In order to flow a forward biascurrent into a light emitting element, the relationship among thevoltage applied to the anode, V2, and V1 is appropriately set. Also, theconstant current source 704 is not limited to the structure as shown inFIG. 29A, a well-known constant current source can be used.

An output terminal of the constant current source 704 is connected witha pixel electrode of the monitoring light emitting element 703. By theway, when an anode of the light emitting element 705 in the pixelportion is used as a pixel electrode, an anode of the monitoring lightemitting element 703 is also used as a pixel electrode. To the contrary,when a cathode of the light emitting element 705 in the pixel portion isused as a pixel electrode, a cathode of the monitoring light emittingelement 703 is also used as a pixel electrode. In FIG. 29A, the anode ofthe monitoring light emitting element 703 is used as a pixel electrode.

When the output terminal of the constant current source 704 is connectedwith the pixel electrode of the monitoring light emitting element 703and thus a current flows into the monitoring light emitting element 703,its value is always kept constant. And then, when a temperature of aorganic compound layer included in the light emitting element ischanged, although an amount of a current flowing into the monitoringlight emitting element 703 is not changed, a potential of the pixelelectrode of the monitoring light emitting element 703, which isconnected with the constant current source 704, is changed.

On the other hand, the buffer amplifier 702 has two input terminals andone output terminal, one of the two input terminals is a non-invertedinput terminal (+), and the other is an inverted input terminal (−). Apotential of the pixel electrode of the monitoring light emittingelement 703 is provided with the non-inverted input terminal of thebuffer amplifier 702.

The buffer amplifier 702 is a circuit for suppressing a change in thepotential of the pixel electrode of the monitoring light emittingelement 703, which is connected with the constant current source 704,due to a load such as a wiring capacitance of the power source line 701or the like. Thus, the potential provided with the non-inverted inputterminal of the buffer amplifier 702 is output from the output terminalwithout the change due to a load such as a wiring capacitance of thepower source line 701 or the like, and provided with the pixel electrodeof the light emitting element 705 in the pixel portion. As a result, acurrent flowing into the monitoring light emitting element 703 is equalto a current flowing into the light emitting element 705 in the pixelportion.

Then, even when a temperature of an organic compound layer of, themonitoring light emitting element 703 or the light emitting element 705in the pixel portion is changed depending on a change in an ambiancetemperature, a constant current flows into each light emitting element.Thus, even when the ambiance temperature of the light emitting devicerises, an increase in power consumption of the light emitting device canbe suppressed.

FIG. 29B shows a change in measurement values of brightness due to atemperature, of the light emitting element 705 in the pixel portion ofthe light emitting device having the structure as shown in FIG. 29A. Bythe way, a graph (corrected) relates to the measurement values in thelight emitting device of the present invention, and a graph (notcorrected) relates to the measurement values in the light emittingdevice without the third structure of the present invention.

As can be cleared from FIG. 29A, in the graph (not corrected), thebrightness is increased depending on the rise of the temperature.However, in the graph (corrected), even when the temperature rises, thebrightness is almost kept constant. Since a current and a brightness arein a proportional relation, in the light emitting device of the presentinvention, even the temperature rises, a current can be kept constant,and the increase in the power consumption can be suppressed.

Also, in a light emitting element, a decrease in brightness is caused bydeterioration of an organic light emitting layer. Here, even if thedeterioration is caused in the same level, a decrease amount inbrightness in the case where a current flowing between a cathode and ananode is kept constant is smaller than that in the case where a voltageapplied between the cathode and the anode is kept constant. Thus, sincea current flowing into the light emitting element can be kept constantin the light emitting device of the present invention, a decrease inbrightness, due to deterioration can be suppressed.

This embodiment can be arbitrarily combined with the structures ofEmbodiments 1 to 9 with respect to the operation.

As described in the above, in accordance with the first structure of thepresent invention, the magnitude of a current to flow through the lightemitting element can be suppressed to some degree, thereby resulting ina reduced power consumption of the light emitting device. In accordancewith the second structure of the present invention, the number of bitsof the digital video signal to be input into the pixels is reduced, andtherefore, the required number of writing the digital video signal bythe source signal line driver circuit and a gate signal line drivercircuit can be reduced. Thus, power consumption of the source signalline driver circuit and the gate signal line driver circuit can bereduced, thereby resulting in a reduced power consumption of the lightemitting device. In accordance with the third structure of the presentinvention, the magnitude of a current to flow through a light emittingelement of a pixel is maintained at a constant level, even when atemperature of the organic compound layer changes. Thus, powerconsumption of the light emitting device can be prevented fromincreasing even when an environmental temperature of the light emittingdevice increases, which in turn can maintain the brightness at aconstant level.

In accordance with the first, second, and third structures of thepresent invention as described in the above, it is possible to suppressthe power consumption of a light emitting device and an electronicapparatus which employs the light emitting device. It should be notedthat only either one of the first through third structures is requiredto be included in the present invention, although two or all of thefirst through third structures may be included.

1. A portable information terminal comprising: a light emitting deviceattached to a main body of the portable information terminal, the lightemitting device comprising: a pixel portion formed over a substrate, thepixel portion having at least one light emitting element, wherein the atleast one light emitting element comprises a first electrode and asecond electrode with a light emitting material interposed therebetween;and a buffer amplifier, a monitoring light emitting element, and aconstant current source, wherein the monitoring light emitting elementand the constant current source are electrically connected to an inputof the buffer amplifier, wherein an output of the buffer amplifier iselectrically connected to the at least one light emitting element, andwherein a brightness change of the at least one light emitting elementcaused by a deterioration of the light emitting material is suppressedby using the monitoring light emitting element.
 2. A portableinformation terminal according to claim 1, wherein the buffer amplifieris formed over an IC chip.
 3. A portable information terminal accordingto claim 1, wherein the portable information terminal is one of aportable telephone and a camera.
 4. A portable information terminalcomprising: a light emitting device attached to a main body of theportable information terminal, the light emitting device comprising: apixel portion formed over a substrate, the pixel portion having at leastone light emitting element, wherein the at least one light emittingelement comprises a first electrode and a second electrode with a lightemitting material interposed therebetween; and a buffer amplifier, amonitoring light emitting element, and a constant current source,wherein the monitoring light emitting element and the constant currentsource are electrically connected to an input of the buffer amplifier,wherein an output of the buffer amplifier is electrically connected tothe at least one light emitting element, and wherein a brightness changeof the at least one light emitting element caused by a temperaturechange of the light emitting material is suppressed by using themonitoring light emitting element.
 5. A portable information terminalaccording to claim 4, wherein the portable information terminal is oneof a portable telephone and a camera.
 6. A portable information terminalcomprising: a light emitting device attached to a main body of theportable information terminal, the light emitting device comprising: apixel portion formed over a substrate, the pixel portion having at leastone light emitting element, wherein the at least one light emittingelement comprises a first electrode and a second electrode with a lightemitting material interposed therebetween; and a buffer amplifier, amonitoring light emitting element, and a constant current source,wherein the monitoring light emitting element and the constant currentsource are electrically connected to an input of the buffer amplifier,wherein an output of the buffer amplifier is electrically connected tothe at least one light emitting element, and wherein a brightnessdecrease of the at least one light emitting element caused by adeterioration of the light emitting material is suppressed by using themonitoring light emitting element.
 7. A portable information terminalaccording to claim 6, wherein the portable information terminal is oneof a portable telephone and a camera.
 8. A portable information terminalcomprising: a light emitting device attached to a main body of theportable information terminal, the light emitting device comprising: apixel portion formed over a substrate, the pixel portion having at leastone light emitting element, wherein the at least one light emittingelement comprises a first electrode and a second electrode with a lightemitting material interposed therebetween; and a buffer amplifier, amonitoring light emitting element, and a constant current source,wherein the monitoring light emitting element and the constant currentsource are electrically connected to an input of the buffer amplifier,wherein an output of the buffer amplifier is electrically connected tothe at least one light emitting element, and wherein a brightnessincrease of the at least one light emitting element caused by atemperature change of the light emitting material is suppressed by usingthe monitoring light emitting element.
 9. A portable informationterminal according to claim 8, wherein the portable information terminalis one of a portable telephone and a camera.
 10. A portable informationterminal comprising: a light emitting device attached to a main body ofthe portable information terminal, the light emitting device comprising:a pixel portion formed over a substrate, the pixel portion having atleast one light emitting element, wherein the at least one lightemitting element comprises a first electrode and a second electrode witha light emitting material interposed therebetween; and a bufferamplifier, a monitoring light emitting element, and a constant currentsource, wherein the monitoring light emitting element and the constantcurrent source are electrically connected to an input of the bufferamplifier, wherein an output of the buffer amplifier is electricallyconnected to the at least one light emitting element through a powersource line, and wherein a brightness change of the at least one lightemitting element caused by a deterioration of the light emittingmaterial is suppressed by using the monitoring light emitting element.11. A portable information terminal according to claim 10, wherein theportable information terminal is one of a portable telephone and acamera.
 12. A portable information terminal comprising: a light emittingdevice attached to a main body of the portable information terminal, thelight emitting device comprising: a pixel portion formed over asubstrate, the pixel portion having at least one light emitting element,wherein the at least one light emitting element comprises a firstelectrode and a second electrode with a light emitting materialinterposed therebetween; and a buffer amplifier, a monitoring lightemitting element, and a constant current source, wherein the monitoringlight emitting element and the constant current source are electricallyconnected to an input of the buffer amplifier, wherein an output of thebuffer amplifier is electrically connected to the at least one lightemitting element through a power source line, and wherein a brightnesschange of the at least one light emitting element caused by atemperature change of the light emitting material is suppressed by usingthe monitoring light emitting element.
 13. A portable informationterminal according to claim 12, wherein the portable informationterminal is one of a portable telephone and a camera.
 14. A portableinformation terminal comprising: a light emitting device attached to amain body of the portable information terminal, the light emittingdevice comprising: a pixel portion formed over a substrate, the pixelportion having at least one light emitting element, wherein the at leastone light emitting element comprises a first electrode and a secondelectrode with a light emitting material interposed therebetween; and abuffer amplifier, a monitoring light emitting element, and a constantcurrent source, wherein the monitoring light emitting element and theconstant current source are electrically connected to an input of thebuffer amplifier, wherein an output of the buffer amplifier iselectrically connected to the at least one light emitting elementthrough a power source line, and wherein a brightness decrease of the atleast one light emitting element caused by a deterioration of the lightemitting material is suppressed by using the monitoring light emittingelement.
 15. A portable information terminal according to claim 14,wherein the portable information terminal is one of a portable telephoneand a camera.
 16. A portable information terminal comprising: a lightemitting device attached to a main body of the portable informationterminal, the light emitting device comprising: a pixel portion formedover a substrate, the pixel portion having at least one light emittingelement, wherein the at least one light emitting element comprises afirst electrode and a second electrode with a light emitting materialinterposed therebetween; and a buffer amplifier, a monitoring lightemitting element, and a constant current source, wherein the monitoringlight emitting element and the constant current source are electricallyconnected to an input of the buffer amplifier, wherein an output of thebuffer amplifier is electrically connected to the at least one lightemitting element through a power source line, and wherein a brightnessincrease of the at least one light emitting element caused by atemperature change of the light emitting material is suppressed by usingthe monitoring light emitting element.
 17. A portable informationterminal according to claim 16, wherein the portable informationterminal is one of a portable telephone and a camera.
 18. A portableinformation terminal comprising: a light emitting device attached to amain body of the portable information terminal, the light emittingdevice comprising: a plurality of pixels, each of the plurality ofpixels including a light emitting element having a pair of electrodes, afirst thin film transistor, a second thin film transistor, a sourcesignal line and a gate signal line, wherein a gate electrode of thefirst thin film transistor is connected to the gate signal line, whereinone of a source region and a drain region of the first thin filmtransistor is connected to the source signal line, and the other one ofthe source region and the drain region of the first thin film transistoris connected to a gate electrode of the second thin film transistor, andwherein one of a source region and a drain region of the second thinfilm transistor is connected to one of the pair of electrodes of thelight emitting element; a constant current source; a monitoring lightemitting element having a pair of electrodes; and a circuit whichoutputs a same potential as an input potential, wherein the constantcurrent source, one of the pair of electrodes of the monitoring lightemitting element, and the circuit are connected with each other, andwherein an output of the circuit is connected to the other one of thesource region and the drain region of the second thin film transistor.19. A portable information terminal according to claim 18, wherein theportable information terminal is one of a portable telephone and acamera.
 20. A portable information terminal comprising: a light emittingdevice attached to a main body of the portable information terminal, thelight emitting device comprising: a plurality of pixels, each of theplurality of pixels including a light emitting element having a pair ofelectrodes, a first thin film transistor, a second thin film transistor,a capacitor, a source signal line and a gate signal line, wherein a gateelectrode of the first thin film transistor is connected to the gatesignal line, wherein one of a source region and a drain region of thefirst thin film transistor is connected to the source signal line, andthe other one of the source region and the drain region of the firstthin film transistor is connected to a gate electrode of the second thinfilm transistor and to one of electrodes of the capacitor, and whereinone of a source region and a drain region of the second thin filmtransistor is connected to one of the pair of electrodes of the lightemitting element; a constant current source; a monitoring light emittingelement having a pair of electrodes; and a circuit which outputs a samepotential as an input potential, wherein the constant current source,one of the pair of electrodes of the monitoring light emitting element,and the circuit are connected with each other, and wherein an output ofthe circuit is connected to the other one of the source region and thedrain region of the second thin film transistor and to the other one ofthe electrodes of the capacitor.
 21. A portable information terminalaccording to claim 20, wherein the portable information terminal is oneof a portable telephone and a camera.
 22. A portable informationterminal comprising: a light emitting device attached to a main body ofthe portable information terminal, the light emitting device comprising:a plurality of pixels, each of the plurality of pixels including a lightemitting element having a pair of electrodes, a first thin filmtransistor, a second thin film transistor, a source signal line, a gatesignal line, and a power source line, wherein a gate electrode of thefirst thin film transistor is connected to the gate signal line, whereinone of a source region and a drain region of the first thin filmtransistor is connected to the source signal line, and the other one ofthe source region and the drain region of the first thin film transistoris connected to a gate electrode of the second thin film transistor, andwherein one of a source region and a drain region of the second thinfilm transistor is connected to one of the pair of electrodes of thelight emitting element, and the other one of the source region and thedrain region of the second thin film transistor is connected to thepower source line; a constant current source; a monitoring lightemitting element having a pair of electrodes; and a circuit whichoutputs a same potential as an input potential, wherein the constantcurrent source, one of the pair of electrodes of the monitoring lightemitting element, and the circuit are connected with each other, andwherein an output of the circuit is connected to the power source line.23. A portable information terminal according to claim 22, wherein theportable information terminal is one of a portable telephone and acamera.
 24. A portable information terminal comprising: a light emittingdevice attached to a main body of the portable information terminal, thelight emitting device comprising: a plurality of pixels, each of theplurality of pixels including a light emitting element having a pair ofelectrodes, a first thin film transistor, a second thin film transistor,a capacitor, a source signal line, a gate signal line, and a powersource line, wherein a gate electrode of the first thin film transistoris connected to the gate signal line, wherein one of a source region anda drain region of the first thin film transistor is connected to thesource signal line, and the other one of the source region and the drainregion of the first thin film transistor is connected to a gateelectrode of the second thin film transistor and to one of electrodes ofthe capacitor, and wherein one of a source region and a drain region ofthe second thin film transistor is connected to one of the pair ofelectrodes of the light emitting element, and the other one of thesource region and the drain region of the second thin film transistor isconnected to the power source line; a constant current source; amonitoring light emitting element having a pair of electrodes; and acircuit which outputs a same potential as an input potential, whereinthe constant current source, one of the pair of electrodes of themonitoring light emitting element, and the circuit are connected witheach other, and wherein an output of the circuit and the other one ofthe electrodes of the capacitor are connected to the power source line.25. A portable information terminal according to claim 24, wherein theportable information terminal is one of a portable telephone and acamera.
 26. A portable information terminal comprising: a light emittingdevice attached to a main body of the portable information terminal, thelight emitting device comprising: a plurality of pixels, each of theplurality of pixels including a light emitting element having a pair ofelectrodes, a first thin film transistor, a second thin film transistor,a source signal line and a gate signal line, wherein a gate electrode ofthe first thin film transistor is connected to the gate signal line,wherein one of a source region and a drain region of the first thin filmtransistor is connected to the source signal line, and the other one ofthe source region and the drain region of the first thin film transistoris connected to a gate electrode of the second thin film transistor, andwherein one of a source region and a drain region of the second thinfilm transistor is connected to one of the pair of electrodes of thelight emitting element; a constant current source; a monitoring lightemitting element having a pair of electrodes; and a buffer amplifier,wherein the constant current source, one of the pair of electrodes ofthe monitoring light emitting element, and the buffer amplifier areconnected with each other, and wherein an output of the buffer amplifieris connected to the other one of the source region and the drain regionof the second thin film transistor.
 27. A portable information terminalaccording to claim 26, wherein the portable information terminal is oneof a portable telephone and a camera.
 28. A portable informationterminal comprising: a light emitting device attached to a main body ofthe portable information terminal, the light emitting device comprising:a plurality of pixels, each of the plurality of pixels including a lightemitting element having a pair of electrodes, a first thin filmtransistor, a second thin film transistor, a capacitor, a source signalline and a gate signal line, wherein a gate electrode of the first thinfilm transistor is connected to the gate signal line, wherein one of asource region and a drain region of the first thin film transistor isconnected to the source signal line, and the other one of the sourceregion and the drain region of the first thin film transistor isconnected to a gate electrode of the second thin film transistor and toone of electrodes of the capacitor, and wherein one of a source regionand a drain region of the second thin film transistor is connected toone of the pair of electrodes of the light emitting element; a constantcurrent source; a monitoring light emitting element having a pair ofelectrodes; and a buffer amplifier, wherein the constant current source,one of the pair of electrodes of the monitoring light emitting element,and the buffer amplifier are connected with each other, and wherein anoutput of the buffer amplifier is connected to the other one of thesource region and the drain region of the second thin film transistorand to the other one of the electrodes of the capacitor.
 29. A portableinformation terminal according to claim 28, wherein the portableinformation terminal is one of a portable telephone and a camera.
 30. Aportable information terminal comprising: a light emitting deviceattached to a main body of the portable information terminal, the lightemitting device comprising: a plurality of pixels, each of the pluralityof pixels including a light emitting element having a pair ofelectrodes, a first thin film transistor, a second thin film transistor,a source signal line, a gate signal line, and a power source line,wherein a gate electrode of the first thin film transistor is connectedto the gate signal line, wherein one of a source region and a drainregion of the first thin film transistor is connected to the sourcesignal line, and the other one of the source region and the drain regionof the first thin film transistor is connected to a gate electrode ofthe second thin film transistor, and wherein one of a source region anda drain region of the second thin film transistor is connected to one ofthe pair of electrodes of the light emitting element, and the other oneof the source region and the drain region of the second thin filmtransistor is connected to the power source line; a constant currentsource; a monitoring light emitting element having a pair of electrodes;and a buffer amplifier, wherein the constant current source, one of thepair of electrodes of the monitoring light emitting element, and thebuffer amplifier are connected with each other, and wherein an output ofthe buffer amplifier is connected to the power source line.
 31. Aportable information terminal according to claim 30, wherein theportable information terminal is one of a portable telephone and acamera.
 32. A portable information terminal comprising: a light emittingdevice attached to a main body of the portable information terminal, thelight emitting device comprising: a plurality of pixels, each of theplurality of pixels including a light emitting element having a pair ofelectrodes, a first thin film transistor, a second thin film transistor,a capacitor, a source signal line, a gate signal line, and a powersource line, wherein a gate electrode of the first thin film transistoris connected to the gate signal line, wherein one of a source region anda drain region of the first thin film transistor is connected to thesource signal line, and the other one of the source region and the drainregion of the first thin film transistor is connected to a gateelectrode of the second thin film transistor and to one of electrodes ofthe capacitor, and wherein one of a source region and a drain region ofthe second thin film transistor is connected to one of the pair ofelectrodes of the light emitting element, and the other one of thesource region and the drain region of the second thin film transistor isconnected to the power source line; a constant current source; amonitoring light emitting element having a pair of electrodes; and abuffer amplifier, wherein the constant current source, one of the pairof electrodes of the monitoring light emitting element, and the bufferamplifier are connected with each other, and wherein an output of thebuffer amplifier and the other one of the electrodes of the capacitorare connected to the power source line.
 33. A portable informationterminal according to claim 32, wherein the portable informationterminal is one of a portable telephone and a camera.